The magic tricks of Houdini A wagic wand for the future of CSS

Logo of CSS Day Faenza, 2019-03-15
My face

Massimo Artizzu

Web dev & architect
at Antreem

Twitter logo / GitHub logo @MaxArt2501

You can find these slides at

QR Code for the presentation's link

Let's talk about…

JavaScript Vincent Vega confused

It gets better!

Let's talk about JavaScript

on Internet Explorer 6

Even Microsoft had good ideas!

David Silverman's meme face Usually, with a very poor execution…

Internet Explorer

introduced 2 ways JavaScript could extend CSS:

IE's CSS expressions

.indicator {
  width: 100%;
  color: expression(this.clientWidth > 400 ? 'red' : 'blue');

IE's custom behaviors

<!-- -->
<component lightWeight="true">
<attach event="onpropertychange" onevent="checkPropertyChange()" />
<attach event="ondetach" onevent="restore()" />
<script type="text/javascript">//<![CDATA[
function checkPropertyChange(){ ... }



Hank Scorpio flamethrowing


What now?

CSS is evolving:

But no matter what the W3C does

CSS will never cover everyone's use cases.

In short…

We're still like this:

Peter Griffin meddling with blinds

So, what can we do?


Enter Houdini

A project inside the W3C to expose parts of the CSS engine to web developers

Houdini modules

Style Layout Paint Composite Rendering cycle
Collaboration effort 🤝


Think of Workers, but lightweight:

Are you ready?

Paint API

What's that for?

Allows to draw an image, to be used as a background, a border, a mask…
.delaunay {
  background-color: #f60;
  background-image: paint(delaunay);
// delaunay.js
class DelaunayModule {
  paint(context, geometry, properties) { ... }

  static get inputProperties() {
    return [ '--delaunay-point-area', ... ];

registerPaint('delaunay', DelaunayModule);

// main.js
Of course the module must adhere to the same-origin policy, must be on https or localhost, yadda yadda…

paint(context, geometry, properties)

context is a stripped-down CanvasRenderingContext2D

👉 geometry has just two numeric properties:

↔ width and ↕ height

👉 properties is a Map of the CSS properties given by inputProperties

Where can I use it?

Some of these behave weirdly, but kinda work. Take note there's must be something to paint.

Typed OM = '20px';
element.attributeStyleMap.set('font-size', CSS.px(20));
Guy looking confused How is that an improvement?!
One word:


and other stuff

(ok, that's 4)

What usually happens

const width = someComputation(); = `${width}px`;

number ➡ string ➡ CSS parse ➡ CSS value

What happens with Typed OM

const width = someComputation();
element.attributeStyleMap.set('width', CSS.px(width));

number ➡ string ➡ CSS parse ➡ CSS value

Style maps

What's .attributeStyleMap?

It's a Map for style properties


It means you can set numbers, strings or typed values; and you get only typed values.

There are two defined on elements:

And by the way

paint(ctx, geom, properties)

That's a StylePropertyMap too.

Other perks

Properties and values

#1: the cascade

.box {
  width: 20em;
  width: red;
width: red gets ignored

What happens now?

.box {
  --box-width: 20em;
  --box-width: red;
  width: var(--box-width);
It's --box-width: 20em that gets ignored!

#2: animations

.box { animation: streeetch 2s infinite; }
@keyframes streeetch {
  from { width: 10em; }
  to   { width: 20em; }
.box {
  width: var(--box-width);
  animation: streeetch 2s infinite;
@keyframes streeetch {
  from { --box-width: 10em; }
  to   { --box-width: 20em; }


width is known to be a length, while --box-width is not

Set the type of a custom property

  name: '--box-width',
  syntax: '<length>',
  initialValue: CSS.px(0),
  inherits: false

Next step 🔭

Register properties in CSS 🎉

@property --box-width {
  syntax: '<length>';
  initial-value: 0px;
  inherits: false;

What we've seen so far

🎨 Paint API

Draw images fast in a worklet

⚡ Typed OM

Fast and less error-prone values for CSS

🔢 Properties and Values

Making custom properties first class citizens

Hello! 👋
registerPaint('ripple', class {
  static get inputProperties() {
    return [ '--ripple-color', ... ];
  paint(ctx, { width, height }, props) {
    const color = props.get('--ripple-color');
    ctx.fillStyle = color;
    ctx.arc(centerX.value, centerY.value,
      radius.value * farthest / 100, 0, Math.PI * 2);
  name: '--ripple-color',
  syntax: '<color>',
  initialValue: '#fff0',
  inherits: false
  name: '--ripple-radius',
  syntax: '<percentage>',
const duration =

  '--ripple-color': '#ffffff80',
  '--ripple-radius': CSS.percent(0)
}, {
  '--ripple-color': '#ffffff00',
  '--ripple-radius': CSS.percent(100)
}], duration);

We can already do a lot, but…

Can I use it in production?

Is Houdini Ready Yet screenshot There's a handy table reporting the status of Houdini's specs at W3C, and their implementation among the major browsers Is Houdini ready Serously, that's the site. Maintained by Surma, the guy who recently talked about Houdini at the last Chrome Dev Summit

Layout API

A house crumbling because of termites
Mock-solid 😰
display: layout(my-layout)
registerLayout('my-layout', class {
  async layout(children, edges, constraints, properties, breakToken) {
  async intrinsicSizes(children, edges, properties) { 🤔 }
  static inputProperties = ['--foo'];
  static childrenInputProperties = ['--bar'];
  static layoutOptions = {childDisplay: 'normal', sizing: 'block-like'};
I won't explain all this, it could be overwhelming. Doing a layout is a very complex task. It would also be futile, as this spec is bound to change.

What can we do?

Easy masonry 🏠

Example of masonry layout

Animation Worklet

CSS Animations
Not really but eh, there's 'animation' in the name
Web Animation API

Web Animation API

Can do a lot…

Could do much more!

registerAnimator('my-animator', class {
  constructor(options = {}) {
    this.options = options;
  animate(currentTime, effect) {
    const newTime = bendTheTime(currentTime);
    effect.localTime = newTime;
await CSS.animationWorklet.addModule('./my-anim.js');

const effect = new KeyframeEffect(el, frames);
const animation = new WorkletAnimation(

Timelines 💫

new Animation(

What is "time"? 🤔

Extend the concept to a cursor in an interval

ScrollTimeline 🎉

const timeline = new ScrollTimeline({
  scrollSource: document.scrollElement,
  orientation: 'vertical',
  timeRange: 1000
We must use it in a worklet animation, though

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Other timelines? 🤞

InputTimeline Not here yet…
Old map of Northern Africa
Hic Sunt Leones

Font metrics API

Layout + font = ❤

An explainer of font metrics

Answering questions


Parser API

Allowing to parse

A literal strawman

Links #1

Links #2

for (const question of questions) {
  await answer(question)