Web Standards

Demystifying TypeScript Discriminated Unions

Css Tricks - Thu, 01/27/2022 - 5:20am

TypeScript is a wonderful tool for writing JavaScript that scales. It’s more or less the de facto standard for the web when it comes to large JavaScript projects. As outstanding as it is, there are some tricky pieces for the unaccustomed. One such area is TypeScript discriminated unions.

Specifically, given this code:

interface Cat { weight: number; whiskers: number; } interface Dog { weight: number; friendly: boolean; } let animal: Dog | Cat;

…many developers are surprised (and maybe even angry) to discover that when they do animal., only the weight property is valid, and not whiskers or friendly. By the end of this post, this will make perfect sense.

Before we dive in, let’s do a quick (and necessary) review of structural typing, and how it differs from nominal typing. This will set up our discussion of TypeScript’s discriminated unions nicely.

Structural typing

The best way to introduce structural typing is to compare it to what it’s not. Most typed languages you’ve probably used are nominally typed. Consider this C# code (Java or C++ would look similar):

class Foo { public int x; } class Blah { public int x; }

Even though Foo and Blah are structured exactly the same, they cannot be assigned to one another. The following code:

Blah b = new Foo();

…generates this error:

Cannot implicitly convert type 'Foo' to 'Blah'

The structure of these classes is irrelevant. A variable of type Foo can only be assigned to instances of the Foo class (or subclasses thereof).

TypeScript operates the opposite way. TypeScript considers types to be compatible if they have the same structure—hence the name, structural typing. Get it?

So, the following runs without error:

class Foo { x: number = 0; } class Blah { x: number = 0; } let f: Foo = new Blah(); let b: Blah = new Foo(); Types as sets of matching values

Let’s hammer this home. Given this code:

class Foo { x: number = 0; } let f: Foo;

f is a variable holding any object that matches the structure of instances created by the Foo class which, in this case, means an x property that represents a number. That means even a plain JavaScript object will be accepted.

let f: Foo; f = { x: 0 } Unions

Thanks for sticking with me so far. Let’s get back to the code from the beginning:

interface Cat { weight: number; whiskers: number; } interface Dog { weight: number; friendly: boolean; }

We know that this:

let animal: Dog;

…makes animal any object that has the same structure as the Dog interface. So what does the following mean?

let animal: Dog | Cat;

This types animal as any object that matches the Dog interface, or any object that matches the Cat interface.

So why does animal—as it exists now—only allow us to access the weight property? To put it simply, it’s because TypeScript does not know which type it is. TypeScript knows that animal has to be either a Dog or Cat, but it could be either (or both at the same time, but let’s keep it simple). We’d likely get runtime errors if we were allowed to access the friendly property, but the instance wound up being a Cat instead of a Dog. Likewise for the whiskers property if the object wound up being a Dog.

Type unions are unions of valid values rather than unions of properties. Developers often write something like this:

let animal: Dog | Cat;

…and expect animal to have the union of Dog and Cat properties. But again, that’s a mistake. This specifies animal as having a value that matches the union of valid Dog values and valid Cat values. But TypeScript will only allow you to access properties it knows are there. For now, that means properties on all the types in the union.


Right now, we have this:

let animal: Dog | Cat;

How do we properly treat animal as a Dog when it’s a Dog, and access properties on the Dog interface, and likewise when it’s a Cat? For now, we can use the in operator. This is an old-school JavaScript operator you probably don’t see very often, but it essentially allows us to test if a property is in an object. Like this:

let o = { a: 12 }; "a" in o; // true "x" in o; // false

It turns out TypeScript is deeply integrated with the in operator. Let’s see how:

let animal: Dog | Cat = {} as any; if ("friendly" in animal) { console.log(animal.friendly); } else { console.log(animal.whiskers); }

This code produces no errors. When inside the if block, TypeScript knows there’s a friendly property, and therefore casts animal as a Dog. And when inside the else block, TypeScript similarly treats animal as a Cat. You can even see this if you hover over the animal object inside these blocks in your code editor:

Discriminated unions

You might expect the blog post to end here but, unfortunately, narrowing type unions by checking for the existence of properties is incredibly limited. It worked well for our trivial Dog and Cat types, but things can easily get more complicated, and more fragile, when we have more types, as well as more overlap between those types.

This is where discriminated unions come in handy. We’ll keep everything the same from before, except add a property to each type whose only job is to distinguish (or “discriminate”) between the types:

interface Cat { weight: number; whiskers: number; ANIMAL_TYPE: "CAT"; } interface Dog { weight: number; friendly: boolean; ANIMAL_TYPE: "DOG"; }

Note the ANIMAL_TYPE property on both types. Don’t mistake this as a string with two different values; this is a literal type. ANIMAL_TYPE: "CAT"; means a type that holds exactly the string "CAT", and nothing else.

And now our check becomes a bit more reliable:

let animal: Dog | Cat = {} as any; if (animal.ANIMAL_TYPE === "DOG") { console.log(animal.friendly); } else { console.log(animal.whiskers); }

Assuming each type participating in the union has a distinct value for the ANIMAL_TYPE property, this check becomes foolproof.

The only downside is that you now have a new property to deal with. Any time you create an instance of a Dog or a Cat, you have to supply the single correct value for the ANIMAL_TYPE. But don’t worry about forgetting because TypeScript will remind you. 🙂

Further reading

If you’d like to learn more, I’d recommend the TypeScript docs on narrowing. That’ll provide some deeper coverage of what we went over here. Inside of that link is a section on type predicates. These allow you to define your own, custom checks to narrow types, without needing to use type discriminators, and without relying on the in keyword.


At the beginning of this article, I said it would make sense why weight is the only accessible property in the following example:

interface Cat { weight: number; whiskers: number; } interface Dog { weight: number; friendly: boolean; } let animal: Dog | Cat;

What we learned is that TypeScript only knows that animal could be either a Dog or a Cat, but not both. As such, all we get is weight, which is the only common property between the two.

The concept of discriminated unions is how TypeScript differentiates between those objects and does so in a way that scales extremely well, even with larger sets of objects. As such, we had to create a new ANIMAL_TYPE property on both types that holds a single literal value we can use to check against. Sure, it’s another thing to track, but it also produces more reliable results—which is what we want from TypeScript in the first place.

Demystifying TypeScript Discriminated Unions originally published on CSS-Tricks. You should get the newsletter and become a supporter.

Build, Ship, & Maintain Design Systems with Backlight

Css Tricks - Thu, 01/27/2022 - 5:18am

(This is a sponsored post.)

Design systems are an entire job these days. Agencies are hired to create them. In-house teams are formed to handle them, shipping them so that other teams can use them and helping ensure they do. Design systems aren’t a fad, they are a positive evolution of how digital design is done. Backlight is the ultimate all-in-one development tool for design systems.

I think it’s interesting to start thinking about this at the end. What’s the best-case scenario for a design system for websites? I think it’s when you’ve published a versioned design system to npm. That way teams can pull it in as a dependency on the project and use it. How do you do that? Your design system is on GitHub and you publish from there. How do you do that? You work on your design system through a development environment that pushes to GitHub. What is Backlight? It’s that development environment.

Spin up a complete design system in seconds

Wanna watch me do it?

You don’t have to pick a starter template, but it’s enlightening to see all the possibilities. Backlight isn’t particularly opinionated about what technology you want to use for the system. Lit and Web Components? Great. React and Emotion? Cool. Just Vue? All good. Nunjucks and Sass? That works.

Having a starter design system really gives you a leg up here. If you’re cool with using something off-the-shelf and then customizing it, you’ll be off and running incredibly quickly. Something that you might assume would take a few weeks to figure out and settle into is done in an instant. And if you want to be 100% custom about everything, that’s still completely on the table.

Kick it up to GitHub

Even if you’re still just testing, I think it’s amazingly easy and impressive how you can just create a GitHub (or GitLab) repo and push to it in a few clicks.

To me, this is the moment it really becomes real. This isn’t some third-party tool where everyone is 100% forced to use it and you’re locked into it forever and it’s only really useful when people buy into the third-party tool. Backlight just takes very industry-standard practices and makes them easier and more convenient to work with.

Then, kick it to a registry.

Like I said at the top, this is the big moment for any design system. When you send it to a package registry like npm or GitHub packages, that means that anyone hoping to use your design system can now install it and use it like any other dependency.

In Backlight, this is just a matter of clicking a few buttons.

With a PRO membership, you can change the scope to your own organization. Soon you’ll be handling all your design system releases right from here, including major, minor, and patch versions.

Make a Component

I’d never used Backlight before, nobody helped me, and I didn’t read any of the (robust) documentation. I just clicked around and created a new Component easily. In my case here, I made a new Nunjucks macro, made some SCSS styles, then created a demo of it as a Storybook “story”. All I did was reference an existing component to see how it all worked.

As one of the creators of CodePen, of course, I highly appreciated the in-browser IDE qualities to all this. It runs re-builds your code changes (looks like a Vite process) super quickly, alerting you helpfully to any errors.

Now because this is a Very Real Serious Design System, I wouldn’t push this new component directly to master in our repository, first it becomes a branch, and then I commit to that. I wouldn’t have to know anything at all about Git to pull this off, look how easy it is:

Howdy, Stakeholders!

Design systems are as much of a people concern as they are a technological concern. Design systems need to get talked about. I really appreciate how I can share Backlight with anyone, even if they aren’t logged in. Just copy a sharing link (that nobody could ever guess) and away you go.

There is a lot here.

You can manage an entire design system in here. You’re managing things from the atomic token level all the way up to building example pages and piecing together the system. You’re literally writing the code to build all this stuff, including the templates, stories, and tests, right there in Backlight.

What about those people on your team who really just can’t be persuaded to leave their local development environment. Backlight understands this, and it doesn’t force them to! Backlight has a CLI which enables local development, including spinning up a server to preview active work.

But it doesn’t stop there. You can build documentation for everything right in Backlight. Design systems are often best explained in words! And design systems might actually start life (or live a parallel life) in entirely design-focused software like Figma, Sketch, or Adobe XD. It’s possible to link design documents right in Backlight, making them easy to find and much more organized.

I’m highly impressed! I wasn’t sure at first what to make of a tool that wants to be a complete tool for design systems, knowing how complex that whole world is, but Backlight really delivers in a way that I find highly satisfying, especially coming at it from the role of a front-end developer, designer, and manager.

Build, Ship, & Maintain Design Systems with Backlight originally published on CSS-Tricks. You should get the newsletter and become a supporter.

How to Cycle Through Classes on an HTML Element

Css Tricks - Wed, 01/26/2022 - 9:48am

Say you have three HTML classes, and a DOM element should only have one of them at a time:

<div class="state-1"></div> <div class="state-2"></div> <div class="state-3"></div>

Now your job is to rotate them. That is, cycle through classes on an HTML element. When some event occurs, if the element has state-1 on it, remove state-1 and add state-2. If it has state-2 on it, remove that and add state-3. On the last state, remove it, and cycle back to state-1.

It’s notable that we’re talking about 3+ classes here. The DOM has a .classList.toggle() function, even one that takes a conditional as a second parameter, but that’s primarily useful in a two-class on/off situation, not cycling through classes.

Why? There is a number of reasons. Changing a class name gives you lots of power to re-style things in the DOM, and state management like that is a cornerstone of modern web development. But to be specific, in my case, I was wanting to do FLIP animations where I’d change a layout and trigger a tween animation between the different states.

Careful about existing classes! I saw some ideas that overwrote .className, which isn’t friendly toward other classes that might be on the DOM element. All these are “safe” choices for cycling through classes in that way.

Because this is programming, there are lots of ways to get this done. Let’s cover a bunch of them — for fun. I tweeted about this issue, so many of these solutions are from people who chimed into that discussion.

A verbose if/else statement to cycle through classes

This is what I did at first to cycle through classes. That’s how my brain works. Just write out very specific instructions for exactly what you want to happen:

if (el.classList.contains("state-1")) { el.classList.remove("state-1"); el.classList.add("state-2"); } else if (el.classList.contains("state-2")) { el.classList.remove("state-2"); el.classList.add("state-3"); } else { el.classList.remove("state-3"); el.classList.add("state-1"); }

I don’t mind the verbosity here, because to me it’s super clear what’s going on and will be easy to return to this code and “reason about it,” as they say. You could consider the verbosity a problem — surely there is a way to cycle through classes with less code. But a bigger issue is that it isn’t very extensible. There is no semblance of configuration (e.g. change the names of the classes easily) or simple way to add classes to the party, or remove them.

We could use constants, at least:

const STATE_1 = "state-1"; const STATE_2 = "state-2"; const STATE_3 = "state-3"; if (el.classList.contains(STATE_1)) { el.classList.remove(STATE_1); el.classList.add(STATE_2); } else if (el.classList.contains(STATE_2)) { el.classList.remove(STATE_2); el.classList.add(STATE_3); } else { el.classList.remove(STATE_3); el.classList.add(STATE_1); }

But that’s not wildly different or better.

RegEx off the old class, increment state, then re-add

This one comes from Tab Atkins. Since we know the format of the class, state-N, we can look for that, pluck off the number, use a little ternary to increment it (but not higher than the highest state), then add/remove the classes as a way of cycling through them:

const oldN = +/\bstate-(\d+)\b/.exec(el.getAttribute('class'))[1]; const newN = oldN >= 3 ? 1 : oldN+1; el.classList.remove(`state-${oldN}`); el.classList.add(`state-${newN}`); Find the index of the class, then remove/add

A bunch of techniques to cycle through classes center around setting up an array of classes up front. This acts as configuration for cycling through classes, which I think is a smart way to do it. Once you have that, you can find the relevant classes for adding and removing them. This one is from Christopher Kirk-Nielsen:

const classes = ["state-1", "state-2", "state-3"]; const activeIndex = classes.findIndex((c) => el.classList.contains(c)); const nextIndex = (activeIndex + 1) % classes.length; el.classList.remove(classes[activeIndex]); el.classList.add(classes[nextIndex]);

Christopher had a nice idea for making the add/remove technique shorter as well. Turns out it’s the same…

el.classList.remove(classes[activeIndex]); el.classList.add(classes[nextIndex]); // Does the same thing. el.classList.replace(classes[activeIndex], classes[nextIndex]);

Mayank had a similar idea for cycling through classes by finding the class in an array, only rather than using classList.contains(), you check the classes currently on the DOM element with what is in the array.

const states = ["state-1", "state-2", "state-3"]; const current = [...el.classList].find(cls => states.includes(cls)); const next = states[(states.indexOf(current) + 1) % states.length]; el.classList.remove(current); el.classList.add(next);

Variations of this were the most common idea. Here’s Jhey’s and here’s Mike Wagz which sets up functions for moving forward and backward.

Cascading replace statements

Speaking of that replace API, Chris Calo had a clever idea where you chain them with the or operator and rely on the fact that it returns true/false if it works or doesn’t. So you do all three and one of them will work!

el.classList.replace("state-1", "state-2") || el.classList.replace("state-2", "state-3") || el.classList.replace("state-3", "state-1");

Nicolò Ribaudo came to the same conclusion.

Just cycle through class numbers

If you pre-configured a 1 upfront, you could cycle through classes 1-3 and add/remove them based on that. This is from Timothy Leverett who lists another similar option in the same tweet.

// Assumes a `let s = 1` upfront el.classList.remove(`state-${s + 1}`); s = (s + 1) % 3; el.classList.add(`state-${s + 1}`); Use data-* attributes instead

Data attributes have the same specificity power, so I have no issue with this. They might actually be more clear in terms of state handling, but even better, they have a special API that makes them nice to manipulate. Munawwar Firoz had an idea that gets this down to a one-liner:

el.dataset.state = (+el.dataset.state % 3) + 1 A data attribute state machine

You can count on David Khourshid to be ready with a state machine:

const simpleMachine = { "1": "2", "2": "3", "3": "1" }; el.dataset.state = simpleMachine[el.dataset.state]; You’ll almost surely want a function

Give yourself a little abstraction, right? Many of the ideas wrote code this way, but so far I’ve move it out to focus on the idea itself. Here, I’ll leave the function in. This one is from Andrea Giammarchi in which a unique function for cycling through classes is set up ahead of time, then you call it as needed:

const rotator = (classes) => ({ classList }) => { const current = classes.findIndex((cls) => classList.contains(cls)); classList.remove(...classes); classList.add(classes[(current + 1) % classes.length]); }; const rotate = rotator(["state-1", "state-2", "state-3"]); rotate(el);

I heard from Kyle Simpson who had this same idea, almost character for character.


There were more ideas in the replies to my original tweet, but are, best I can tell, variations on what I’ve already shared above. Apologies if I missed yours! Feel free to share your idea again in the comments here. I see nobody used a switch statements — that could be a possibility!

David Desandro went as far as recording a video, which is wonderful as it slowly abstracts the concepts further and further until it’s succinct but still readable and much more flexible:

And here’s a demo Pen with all the code for each example in there. They are numbered, so to test out another one, comment out the one that is uncommented, and uncomment another example:

CodePen Embed Fallback

How to Cycle Through Classes on an HTML Element originally published on CSS-Tricks. You should get the newsletter and become a supporter.

New business wanted

QuirksBlog - Thu, 09/30/2021 - 12:22am

Last week Krijn and I decided to cancel performance.now() 2021. Although it was the right decision it leaves me in financially fairly dire straits. So I’m looking for new jobs and/or donations.

Even though the Corona trends in NL look good, and we could probably have brought 350 people together in November, we cannot be certain: there might be a new flare-up. More serious is the fact that it’s very hard to figure out how to apply the Corona checks Dutch government requires, especially for non-EU citizens. We couldn’t figure out how UK and US people should be tested, and for us that was the straw that broke the camel’s back. Cancelling the conference relieved us of a lot of stress.

Still, it also relieved me of a lot of money. This is the fourth conference in a row we cannot run, and I have burned through all my reserves. That’s why I thought I’d ask for help.

So ...

Has QuirksMode.org ever saved you a lot of time on a project? Did it advance your career? If so, now would be a great time to make a donation to show your appreciation.

I am trying my hand at CSS coaching. Though I had only few clients so far I found that I like it and would like to do it more. As an added bonus, because I’m still writing my CSS for JavaScripters book I currently have most of the CSS layout modules in my head and can explain them straight away — even stacking contexts.

Or if there’s any job you know of that requires a technical documentation writer with a solid knowledge of web technologies and the browser market, drop me a line. I’m interested.

Anyway, thanks for listening.

position: sticky, draft 1

QuirksBlog - Wed, 09/08/2021 - 7:44am

I’m writing the position: sticky part of my book, and since I never worked with sticky before I’m not totally sure if what I’m saying is correct.

This is made worse by the fact that there are no very clear tutorials on sticky. That’s partly because it works pretty intuitively in most cases, and partly because the details can be complicated.

So here’s my draft 1 of position: sticky. There will be something wrong with it; please correct me where needed.

The inset properties are top, right, bottom and left. (I already introduced this terminology earlier in the chapter.)

h3,h4,pre {clear: left} section.scroll-container { border: 1px solid black; width: 300px; height: 250px; padding: 1em; overflow: auto; --text: 'scroll box'; float: left; clear: left; margin-right: 0.5em; margin-bottom: 1em; position: relative; font-size: 1.3rem; } .container,.outer-container { border: 1px solid black; padding: 1em; position: relative; --text: 'container'; } .outer-container { --text: 'outer container'; } :is(.scroll-container,.container,.outer-container):before { position: absolute; content: var(--text); top: 0.2em; left: 0.2em; font-size: 0.8rem; } section.scroll-container h2 { position: sticky; top: 0; background: white; margin: 0 !important; color: inherit !important; padding: 0.5em !important; border: 1px solid; font-size: 1.4rem !important; } .nowrap p { white-space: nowrap; } Introduction

position: sticky is a mix of relative and fixed. A sticky box takes its normal position in the flow, as if it had position: relative, but if that position scrolls out of view the sticky box remains in a position defined by its inset properties, as if it has position: fixed. A sticky box never escapes its container, though. If the container start or end scrolls past the sticky box abandons its fixed position and sticks to the top or the bottom of its container.

It is typically used to make sure that headers remain in view no matter how the user scrolls. It is also useful for tables on narrow screens: you can keep headers or the leftmost table cells in view while the user scrolls.

Scroll box and container

A sticky box needs a scroll box: a box that is able to scroll. By default this is the browser window — or, more correctly, the layout viewport — but you can define another scroll box by setting overflow on the desired element. The sticky box takes the first ancestor that could scroll as its scroll box and calculates all its coordinates relative to it.

A sticky box needs at least one inset property. These properties contain vital instructions, and if the sticky box doesn’t receive them it doesn’t know what to do.

A sticky box may also have a container: a regular HTML element that contains the sticky box. The sticky box will never be positioned outside this container, which thus serves as a constraint.

The first example shows this set-up. The sticky <h2> is in a perfectly normal <div>, its container, and that container is in a <section> that is the scroll box because it has overflow: auto. The sticky box has an inset property to provide instructions. The relevant styles are:

section.scroll-container { border: 1px solid black; width: 300px; height: 300px; overflow: auto; padding: 1em; } div.container { border: 1px solid black; padding: 1em; } section.scroll-container h2 { position: sticky; top: 0; } The rules Sticky header

Regular content

Regular content

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Content outside container

Content outside container

Content outside container

Content outside container

Content outside container

Content outside container

Now let’s see exactly what’s going on.

A sticky box never escapes its containing box. If it cannot obey the rules that follow without escaping from its container, it instead remains at the edge. Scroll down until the container disappears to see this in action.

A sticky box starts in its natural position in the flow, as if it has position: relative. It thus participates in the default flow: if it becomes higher it pushes the paragraphs below it downwards, just like any other regular HTML element. Also, the space it takes in the normal flow is kept open, even if it is currently in fixed position. Scroll down a little bit to see this in action: an empty space is kept open for the header.

A sticky box compares two positions: its natural position in the flow and its fixed position according to its inset properties. It does so in the coordinate frame of its scroll box. That is, any given coordinate such as top: 20px, as well as its default coordinates, is resolved against the content box of the scroll box. (In other words, the scroll box’s padding also constrains the sticky box; it will never move up into that padding.)

A sticky box with top takes the higher value of its top and its natural position in the flow, and positions its top border at that value. Scroll down slowly to see this in action: the sticky box starts at its natural position (let’s call it 20px), which is higher than its defined top (0). Thus it rests at its position in the natural flow. Scrolling up a few pixels doesn’t change this, but once its natural position becomes less than 0, the sticky box switches to a fixed layout and stays at that position.

The sticky box has bottom: 0

Regular content

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Sticky header

Content outside container

Content outside container

Content outside container

Content outside container

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It does the same for bottom, but remember that a bottom is calculated relative to the scroll box’s bottom, and not its top. Thus, a larger bottom coordinate means the box is positioned more to the top. Now the sticky box compares its default bottom with the defined bottom and uses the higher value to position its bottom border, just as before.

With left, it uses the higher value of its natural position and to position its left border; with right, it does the same for its right border, bearing in mind once more that a higher right value positions the box more to the left.

If any of these steps would position the sticky box outside its containing box it takes the position that just barely keeps it within its containing box.

Details Sticky header

Very, very long line of content to stretch up the container quite a bit

Regular content

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Content outside container

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The four inset properties act independently of one another. For instance the following box will calculate the position of its top and left edge independently. They can be relative or fixed, depending on how the user scrolls.

p.testbox { position: sticky; top: 0; left: 0; }

Content outside container

Content outside container

Content outside container

Content outside container

Content outside container

The sticky box has top: 0; bottom: 0

Regular content

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Sticky header

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Content outside container

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Setting both a top and a bottom, or both a left and a right, gives the sticky box a bandwidth to move in. It will always attempt to obey all the rules described above. So the following box will vary between 0 from the top of the screen to 0 from the bottom, taking its default position in the flow between these two positions.

p.testbox { position: sticky; top: 0; bottom: 0; } No container

Regular content

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Sticky header

Regular content

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So far we put the sticky box in a container separate from the scroll box. But that’s not necessary. You can also make the scroll box itself the container if you wish. The sticky element is still positioned with respect to the scroll box (which is now also its container) and everything works fine.

Several containers Sticky header

Regular content

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Content outside container

Content outside container

Content outside outer container

Content outside outer container

Or the sticky item can be several containers removed from its scroll box. That’s fine as well; the positions are still calculated relative to the scroll box, and the sticky box will never leave its innermost container.

Changing the scroll box Sticky header

The container has overflow: auto.

Regular content

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Content outside container

Content outside container

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One feature that catches many people (including me) unaware is giving the container an overflow: auto or hidden. All of a sudden it seems the sticky header doesn’t work any more.

What’s going on here? An overflow value of auto, hidden, or scroll makes an element into a scroll box. So now the sticky box’s scroll box is no longer the outer element, but the inner one, since that is now the closest ancestor that is able to scroll.

The sticky box appears to be static, but it isn’t. The crux here is that the scroll box could scroll, thanks to its overflow value, but doesn’t actually do so because we didn’t give it a height, and therefore it stretches up to accomodate all of its contents.

Thus we have a non-scrolling scroll box, and that is the root cause of our problems.

As before, the sticky box calculates its position by comparing its natural position relative to its scroll box with the one given by its inset properties. Point is: the sticky box doesn’t scroll relative to its scroll box, so its position always remains the same. Where in earlier examples the position of the sticky element relative to the scroll box changed when we scrolled, it no longer does so, because the scroll box doesn’t scroll. Thus there is no reason for it to switch to fixed positioning, and it stays where it is relative to its scroll box.

The fact that the scroll box itself scrolls upward is irrelevant; this doesn’t influence the sticky box in the slightest.

Sticky header

Regular content

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Regular content

Regular content

Content outside container

Content outside container

Content outside container

Content outside container

Content outside container

Content outside container

One solution is to give the new scroll box a height that is too little for its contents. Now the scroll box generates a scrollbar and becomes a scrolling scroll box. When we scroll it the position of the sticky box relative to its scroll box changes once more, and it switches from fixed to relative or vice versa as required.

Minor items

Finally a few minor items:

  • It is no longer necessary to use position: -webkit-sticky. All modern browsers support regular position: sticky. (But if you need to cater to a few older browsers, retaining the double syntax doesn’t hurt.)
  • Chrome (Mac) does weird things to the borders of the sticky items in these examples. I don’t know what’s going on and am not going to investigate.

Breaking the web forward

QuirksBlog - Thu, 08/12/2021 - 5:19am

Safari is holding back the web. It is the new IE, after all. In contrast, Chrome is pushing the web forward so hard that it’s starting to break. Meanwhile web developers do nothing except moan and complain. The only thing left to do is to pick our poison.

blockquote { font-size: inherit; font-family: inherit; } blockquote p { font-size: inherit; font-family: inherit; } Safari is the new IE

Recently there was yet another round of “Safari is the new IE” stories. Once Jeremy’s summary and a short discussion cleared my mind I finally figured out that Safari is not IE, and that Safari’s IE-or-not-IE is not the worst problem the web is facing.

Perry Sun argues that for developers, Safari is crap and outdated, emulating the old IE of fifteen years ago in this respect. He also repeats the theory that Apple is deliberately starving Safari of features in order to protect the app store, and thus its bottom line. We’ll get back to that.

The allegation that Safari is holding back web development by its lack of support for key features is not new, but it’s not true, either. Back fifteen years ago IE held back the web because web developers had to cater to its outdated technology stack. “Best viewed with IE” and all that. But do you ever see a “Best viewed with Safari” notice? No, you don’t. Another browser takes that special place in web developers’ hearts and minds.

Chrome is the new IE, but in reverse

Jorge Arango fears we’re going back to the bad old days with “Best viewed in Chrome.” Chris Krycho reinforces this by pointing out that, even though Chrome is not the standard, it’s treated as such by many web developers.

“Best viewed in Chrome” squares very badly with “Safari is the new IE.” Safari’s sad state does not force web developers to restrict themselves to Safari-supported features, so it does not hold the same position as IE.

So I propose to lay this tired old meme to rest. Safari is not the new IE. If anything it’s the new Netscape 4.

Meanwhile it is Chrome that is the new IE, but in reverse.

Break the web forward

Back in the day, IE was accused of an embrace, extend, and extinguish strategy. After IE6 Microsoft did nothing for ages, assuming it had won the web. Thanks to web developers taking action in their own name for the first (and only) time, IE was updated once more and the web moved forward again.

Google learned from Microsoft’s mistakes and follows a novel embrace, extend, and extinguish strategy by breaking the web and stomping on the bits. Who cares if it breaks as long as we go forward. And to hell with backward compatibility.

Back in 2015 I proposed to stop pushing the web forward, and as expected the Chrome devrels were especially outraged at this idea. It never went anywhere. (Truth to tell: I hadn’t expected it to.)

I still think we should stop pushing the web forward for a while until we figure out where we want to push the web forward to — but as long as Google is in charge that won’t happen. It will only get worse.

On alert

A blog storm broke out over the decision to remove alert(), confirm() and prompt(), first only the cross-origin variants, but eventually all of them. Jeremy and Chris Coyier already summarised the situation, while Rich Harris discusses the uses of the three ancient modals, especially when it comes to learning JavaScript.

With all these articles already written I will only note that, if the three ancient modals are truly as horrendous a security issue as Google says they are it took everyone a bloody long time to figure that out. I mean, they turn 25 this year.

Although it appears Firefox and Safari are on board with at least the cross-origin part of the proposal, there is no doubt that it’s Google that leads the charge.

From Google’s perspective the ancient modals have one crucial flaw quite apart from their security model: they weren’t invented there. That’s why they have to be replaced by — I don’t know what, but it will likely be a very complicated API.

Complex systems and arrogant priests rule the web

Thus the new embrace, extend, and extinguish is breaking backward compatibility in order to make the web more complicated. Nolan Lawson puts it like this:

we end up with convoluted specs like Service Worker that you need a PhD to understand, and yet we still don't have a working <dialog> element.

In addition, Google can be pretty arrogant and condescending, as Chris Ferdinandi points out.

The condescending “did you actually read it, it’s so clear” refrain is patronizing AF. It’s the equivalent of “just” or “simply” in developer documentation.

I read it. I didn’t understand it. That’s why I asked someone whose literal job is communicating with developers about changes Chrome makes to the platform.

This is not isolated to one developer at Chrome. The entire message thread where this change was surfaced is filled with folks begging Chrome not to move forward with this proposal because it will break all-the-things.

If you write documentation or a technical article and nobody understands it, you’ve done a crappy job. I should know; I’ve been writing this stuff for twenty years.

Extend, embrace, extinguish. And use lots of difficult words.

Patience is a virtue

As a reaction to web dev outcry Google temporarily halted the breaking of the web. That sounds great but really isn’t. It’s just a clever tactical move.

I saw this tactic in action before. Back in early 2016 Google tried to break the de-facto standard for the mobile visual viewport that I worked very hard to establish. I wrote a piece that resonated with web developers, whose complaints made Google abandon the plan — temporarily. They tried again in late 2017, and I again wrote an article, but this time around nobody cared and the changes took effect and backward compatibility was broken.

So the three ancient modals still have about 12 to 18 months to live. Somewhere in late 2022 to early 2023 Google will try again, web developers will be silent, and the modals will be gone.

The pursuit of appiness

But why is Google breaking the web forward at such a pace? And why is Apple holding it back?

Safari is kept dumb to protect the app store and thus revenue. In contrast, the Chrome team is pushing very hard to port every single app functionality to the browser. Ages ago I argued we should give up on this, but of course no one listened.

When performing Valley Kremlinology, it is useful to see Google policies as stemming from a conflict between internal pro-web and anti-web factions. We web developers mainly deal with the pro-web faction, the Chrome devrel and browser teams. On the other hand, the Android team is squarely in the anti-web camp.

When seen in this light the pro-web camp’s insistence on copying everything appy makes excellent sense: if they didn’t Chrome would lag behind apps and the Android anti-web camp would gain too much power. While I prefer the pro-web over the anti-web camp, I would even more prefer the web not to be a pawn in an internal Google power struggle. But it has come to that, no doubt about it.


Is there any good solution? Not really.

Jim Nielsen feels that part of the issue is the lack of representation of web developers in the standardization process. That sounds great but is proven not to work.

Three years ago Fronteers and I attempted to get web developers represented and were met with absolute disinterest. Nobody else cared even one shit, and the initiative sank like a stone.

So a hypothetical web dev representative in W3C is not going to work. Also, the organisational work would involve a lot of unpaid labour, and I, for one, am not willing to do it again. Neither is anyone else. So this is not the solution.

And what about Firefox? Well, what about it? Ten years ago it made a disastrous mistake by ignoring the mobile web for way too long, then it attempted an arrogant and uninformed come-back with Firefox OS that failed, and its history from that point on is one long slide into obscurity. That’s what you get with shitty management.

Pick your poison

So Safari is trying to slow the web down. With Google’s move-fast-break-absofuckinglutely-everything axiom in mind, is Safari’s approach so bad?

Regardless of where you feel the web should be on this spectrum between Google and Apple, there is a fundamental difference between the two.

We have the tools and procedures to manage Safari’s disinterest. They’re essentially the same as the ones we deployed against Microsoft back in the day — though a fundamental difference is that Microsoft was willing to talk while Apple remains its old haughty self, and its “devrels” aren’t actually allowed to do devrelly things such as managing relations with web developers. (Don’t blame them, by the way. If something would ever change they’re going to be our most valuable internal allies — just as the IE team was back in the day.)

On the other hand, we have no process for countering Google’s reverse embrace, extend, and extinguish strategy, since a section of web devs will be enthusiastic about whatever the newest API is. Also, Google devrels talk. And talk. And talk. And provide gigs of data that are hard to make sense of. And refer to their proprietary algorithms that “clearly” show X is in the best interest of the web — and don’t ask questions! And make everything so fucking complicated that we eventually give up and give in.

So pick your poison. Shall we push the web forward until it’s broken, or shall we break it by inaction? What will it be? Privately, my money is on Google. So we should say goodbye to the old web while we still can.

Custom properties and @property

QuirksBlog - Wed, 07/21/2021 - 3:18am

You’re reading a failed article. I hoped to write about @property and how it is useful for extending CSS inheritance considerably in many different circumstances. Alas, I failed. @property turns out to be very useful for font sizes, but does not even approach the general applicability I hoped for.


It all started when I commented on what I thought was an interesting but theoretical idea by Lea Verou: what if elements could inherit the font size of not their parent, but their grandparent? Something like this:

div.grandparent { /* font-size could be anything */ } div.parent { font-size: 0.4em; } div.child { font-size: [inherit from grandparent in some sort of way]; font-size: [yes, you could do 2.5em to restore the grandparent's font size]; font-size: [but that's not inheriting, it's just reversing a calculation]; font-size: [and it will not work if the parent's font size is also unknown]; }

Lea told me this wasn’t a vague idea, but something that can be done right now. I was quite surprised — and I assume many of my readers are as well — and asked for more information. So she wrote Inherit ancestor font-size, for fun and profit, where she explained how the new Houdini @property can be used to do this.

This was seriously cool. Also, I picked up a few interesting bits about how CSS custom properties and Houdini @property work. I decided to explain these tricky bits in simple terms — mostly because I know that by writing an explanation I myself will understand them better — and to suggest other possibilities for using Lea’s idea.

Alas, that last objective is where I failed. Lea’s idea can only be used for font sizes. That’s an important use case, but I had hoped for more. The reasons why it doesn’t work elsewhere are instructive, though.

Tokens and values

Let’s consider CSS custom properties. What if we store the grandparent’s font size in a custom property and use that in the child?

div.grandparent { /* font-size could be anything */ --myFontSize: 1em; } div.parent { font-size: 0.4em; } div.child { font-size: var(--myFontSize); /* hey, that's the grandparent's font size, isn't it? */ }

This does not work. The child will have the same font size as the parent, and ignore the grandparent. In order to understand why we need to understand how custom properties work. What does this line of CSS do?

--myFontSize: 1em;

It sets a custom property that we can use later. Well duh.

Sure. But what value does this custom property have?

... errr ... 1em?

Nope. The answer is: none. That’s why the code example doesn’t work.

When they are defined, custom properties do not have a value or a type. All that you ordered the browsers to do is to store a token in the variable --myFontSize.

This took me a while to wrap my head around, so let’s go a bit deeper. What is a token? Let’s briefly switch to JavaScript to explain.

let myVar = 10;

What’s the value of myVar in this line? I do not mean: what value is stored in the variable myVar, but: what value does the character sequence myVar have in that line of code? And what type?

Well, none. Duh. It’s not a variable or value, it’s just a token that the JavaScript engine interprets as “allow me to access and change a specific variable” whenever you type it.

CSS custom properties also hold such tokens. They do not have any intrinsic meaning. Instead, they acquire meaning when they are interpreted by the CSS engine in a certain context, just as the myVar token is in the JavaScript example.

So the CSS custom property contains the token 1em without any value, without any type, without any meaning — as yet.

You can use pretty any bunch of characters in a custom property definition. Browsers make no assumptions about their validity or usefulness because they don’t yet know what you want to do with the token. So this, too, is a perfectly fine CSS custom property:

--myEgoTrip: ppk;

Browsers shrug, create the custom property, and store the indicated token. The fact that ppk is invalid in all CSS contexts is irrelevant: we haven’t tried to use it yet.

It’s when you actually use the custom property that values and types are assigned. So let’s use it:

background-color: var(--myEgoTrip);

Now the CSS parser takes the tokens we defined earlier and replaces the custom property with them:

background-color: ppk;

And only NOW the tokens are read and intrepreted. In this case that results in an error: ppk is not a valid value for background-color. So the CSS declaration as a whole is invalid and nothing happens — well, technically it gets the unset value, but the net result is the same. The custom property itself is still perfectly valid, though.

The same happens in our original code example:

div.grandparent { /* font-size could be anything */ --myFontSize: 1em; /* just a token; no value, no meaning */ } div.parent { font-size: 0.4em; } div.child { font-size: var(--myFontSize); /* becomes */ font-size: 1em; /* hey, this is valid CSS! */ /* Right, you obviously want the font size to be the same as the parent's */ /* Sure thing, here you go */ }

In div.child he tokens are read and interpreted by the CSS parser. This results in a declaration font-size: 1em;. This is perfectly valid CSS, and the browsers duly note that the font size of this element should be 1em.

font-size: 1em is relative. To what? Well, to the parent’s font size, of course. Duh. That’s how CSS font-size works.

So now the font size of the child becomes the same as its parent’s, and browsers will proudly display the child element’s text in the same font size as the parent element’s while ignoring the grandparent.

This is not what we wanted to achieve, though. We want the grandparent’s font size. Custom properties — by themselves — don’t do what we want. We have to find another solution.


Lea’s article explains that other solution. We have to use the Houdini @property rule.

@property --myFontSize { syntax: "<length>"; initial-value: 0; inherits: true; } div { border: 1px solid; padding: 1em; } div.grandparent { /* font-size could be anything */ --myFontSize: 1em; } div.parent { font-size: 0.4em; } div.child { font-size: var(--myFontSize); }

Now it works. Wut? Yep — though only in Chrome so far.

@property --myFontSize { syntax: ""; initial-value: 0; inherits: true; } section.example { max-width: 500px; } section.example div { border: 1px solid; padding: 1em; } div.grandparent { font-size: 23px; --myFontSize: 1em; } div.parent { font-size: 0.4em; } div.child { font-size: var(--myFontSize); } This is the grandparent This is the parent This is the child

What black magic is this?

Adding the @property rule changes the custom property --myFontSize from a bunch of tokens without meaning to an actual value. Moreover, this value is calculated in the context it is defined in — the grandfather — so that the 1em value now means 100% of the font size of the grandfather. When we use it in the child it still has this value, and therefore the child gets the same font size as the grandfather, which is exactly what we want to achieve.

(The variable uses a value from the context it’s defined in, and not the context it’s executed in. If, like me, you have a grounding in basic JavaScript you may hear “closures!” in the back of your mind. While they are not the same, and you shouldn’t take this apparent equivalency too far, this notion still helped me understand. Maybe it’ll help you as well.)

Unfortunately I do not quite understand what I’m doing here, though I can assure you the code snippet works in Chrome — and will likely work in the other browsers once they support @property.

Misson completed — just don’t ask me how.


You have to get the definition right. You need all three lines in the @property rule. See also the specification and the MDN page.

@property --myFontSize { syntax: "<length>"; initial-value: 0; inherits: true; }

The syntax property tells browsers what kind of property it is and makes parsing it easier. Here is the list of possible values for syntax, and in 99% of the cases one of these values is what you need.

You could also create your own syntax, e.g. syntax: "ppk | <length>"

Now the ppk keyword and any sort of length is allowed as a value.

Note that percentages are not lengths — one of the many things I found out during the writing of this article. Still, they are so common that a special value for “length that may be a percentage or may be calculated using percentages” was created:

syntax: "<length-percentage>"

Finally, one special case you need to know about is this one:

syntax: "*"

MDN calls this a universal selector, but it isn’t, really. Instead, it means “I don’t know what syntax we’re going to use” and it tells browsers not to attempt to interpret the custom property. In our case that would be counterproductive: we definitely want the 1em to be interpreted. So our example doesn’t work with syntax: "*".

initial-value and inherits

An initial-value property is required for any syntax value that is not a *. Here that’s simple: just give it an initial value of 0 — or 16px, or any absolute value. The value doesn’t really matter since we’re going to overrule it anyway. Still, a relative value such as 1em is not allowed: browsers don’t know what the 1em would be relative to and reject it as an initial value.

Finally, inherits: true specifies that the custom property value can be inherited. We definitely want the computed 1em value to be inherited by the child — that’s the entire point of this experiment. So we carefully set this flag to true.

Other use cases

So far this article merely rehashed parts of Lea’s. Since I’m not in the habit of rehashing other people’s articles my original plan was to add at least one other use case. Alas, I failed, though Lea was kind enough to explain why each of my ideas fails.

Percentage of what?

Could we grandfather-inherit percentual margins and paddings? They are relative to the width of the parent of the element you define them on, and I was wondering if it might be useful to send the grandparent’s margin on to the child just like the font size. Something like this:

@property --myMargin { syntax: "<length-percentage>"; initial-value: 0; inherits: true; } div.grandparent { --myMargin: 25%; margin-left: var(--myMargin); } div.parent { font-size: 0.4em; } div.child { margin-left: var(--myMargin); /* should now be 25% of the width of the grandfather's parent */ /* but isn't */ }

Alas, this does not work. Browsers cannot resolve the 25% in the context of the grandparent, as they did with the 1em, because they don’t know what to do.

The most important trick for using percentages in CSS is to always ask yourself: “percentage of WHAT?”

That’s exactly what browsers do when they encounter this @property definition. 25% of what? The parent’s font size? Or the parent’s width? (This is the correct answer, but browsers have no way of knowing that.) Or maybe the width of the element itself, for use in background-position?

Since browsers cannot figure out what the percentage is relative to they do nothing: the custom property gets the initial value of 0 and the grandfather-inheritance fails.


Another idea I had was using this trick for the grandfather’s text colour. What if we store currentColor, which always has the value of the element’s text colour, and send it on to the grandchild? Something like this:

@property --myColor { syntax: "<color>"; initial-value: black; inherits: true; } div.grandparent { /* color unknown */ --myColor: currentColor; } div.parent { color: red; } div.child { color: var(--myColor); /* should now have the same color as the grandfather */ /* but doesn't */ }

Alas, this does not work either. When the @property blocks are evaluated, and 1em is calculated, currentColor specifically is not touched because it is used as an initial (default) value for some inherited SVG and CSS properties such as fill. Unfortunately I do not fully understand what’s going on, but Tab says this behaviour is necessary, so it is.

Pity, but such is life. Especially when you’re working with new CSS functionalities.


So I tried to find more possbilities for using Lea’s trick, but failed. Relative units are fairly sparse, especially when you leave percentages out of the equation. em and related units such as rem are the only ones, as far as I can see.

So we’re left with a very useful trick for font sizes. You should use it when you need it (bearing in mind that right now it’s only supported in Chromium-based browsers), but extending it to other declarations is not possible at the moment.

Many thanks to Lea Verou and Tab Atkins for reviewing and correcting an earlier draft of this article.

Let&#8217;s talk about money

QuirksBlog - Tue, 06/29/2021 - 1:23am

Let’s talk about money!

Let’s talk about how hard it is to pay small amounts online to people whose work you like and who could really use a bit of income. Let’s talk about how Coil aims to change that.

Taking a subscription to a website is moderately easy, but the person you want to pay must have enabled them. Besides, do you want to purchase a full subscription in order to read one or two articles per month?

Sending a one-time donation is pretty easy as well, but, again, the site owner must have enabled them. And even then it just gives them ad-hoc amounts that they cannot depend on.

Then there’s Patreon and Kickstarter and similar systems, but Patreon is essentially a subscription service while Kickstarter is essentially a one-time donation service, except that both keep part of the money you donate.

And then there’s ads ... Do we want small content creators to remain dependent on ads and thus support the entire ad ecosystem? I, personally, would like to get rid of them.

The problem today is that all non-ad-based systems require you to make conscious decisions to support someone — and even if you’re serious about supporting them you may forget to send in a monthly donation or to renew your subscription. It sort-of works, but the user experience can be improved rather dramatically.

That’s where Coil and the Web Monetization Standard come in.

Web Monetization

The idea behind Coil is that you pay for what you consume easily and automatically. It’s not a subscription - you only pay for what you consume. It’s not a one-time donation, either - you always pay when you consume.

Payments occur automatically when you visit a website that is also subscribed to Coil, and the amount you pay to a single site owner depends on the time you spend on the site. Coil does not retain any of your money, either — everything goes to the people you support.

In this series of four articles we’ll take a closer look at the architecture of the current Coil implementation, how to work with it right now, the proposed standard, and what’s going to happen in the future.


So how does Coil work right now?

Both the payer and the payee need a Coil account to send and receive money. The payee has to add a <meta> tag with a Coil payment pointer to all pages they want to monetize. The payer has to install the Coil extension in their browsers. You can see this extension as a polyfill. In the future web monetization will, I hope, be supported natively in all browsers.

Once that’s done the process works pretty much automatically. The extension searches for the <meta> tag on any site the user visits. If it finds one it starts a payment stream from payer to payee that continues for as long as the payer stays on the site.

The payee can use the JavaScript API to interact with the monetization stream. For instance, they can show extra content to paying users, or keep track of how much a user paid so far. Unfortunately these functionalities require JavaScript, and the hiding of content is fairly easy to work around. Thus it is not yet suited for serious business purposes, especially in web development circles.

This is one example of how the current system is still a bit rough around the edges. You’ll find more examples in the subsequent articles. Until the time browsers support the standard natively and you can determine your visitors’ monetization status server-side these rough bits will continue to exist. For the moment we will have to work with the system we have.

This article series will discuss all topics we touched on in more detail.

Start now!

For too long we have accepted free content as our birthright, without considering the needs of the people who create it. This becomes even more curious for articles and documentation that are absolutely vital to our work as web developers.

Take a look at this list of currently-monetized web developer sites. Chances are you’ll find a few people whose work you used in the past. Don’t they deserve your direct support?

Free content is not a right, it’s an entitlement. The sooner we internalize this, and start paying independent voices, the better for the web.

The only alternative is that all articles and documentation that we depend on will written by employees of large companies. And employees, no matter how well-meaning, will reflect the priorities and point of view of their employer in the long run.

So start now.

In order to support them you should invest a bit of time once and US$5 per month permanently. I mean, that’s not too much to ask, is it?


I wrote this article and its sequels for Coil, and yes, I’m getting paid. Still, I believe in what they are doing, so I won’t just spread marketing drivel. Initially it was unclear to me exactly how Coil works. So I did some digging, and the remaining parts of this series give a detailed description of how Coil actually works in practice.

For now the other three articles will only be available on dev.to. I just published part 2, which gives a high-level overview of how Coil works right now. Part 3 will describe the meta tag and the JavaScript API, and in part 4 we’ll take a look at the future, which includes a formal W3C standard. Those parts will be published next week and the week after that.

Wed, 12/31/1969 - 2:00pm
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