CSS is a lot of things. Frustrating to newcomers, essential (even admirable) to those of us who use it day-to-day. It’s so intertwined with the browser’s rendering model that it can be hard to figure out what’s CSS and what’s the browser. And as most of us know from experience, it’s extremely easy to build an unmaintainable mess with it.

Whatever you think of CSS, it’s demonstrably underspecified. (edit: one of the CSS authoring committee rightly pointed out that the correct word is incomplete. CSS as it stands is unambiguous, and implementations in modern browsers don’t differ, but it doesn’t cover everything the users want. That’s not underspecification.) Gaps in the language itself gave rise to a host of pre-processors, polyfills & workarounds, and the lack of any true runtime isolation resulted in a long line of conventions & techniques to keep code maintainable. Until the spec catches up (CSS Variables, Color functions, Custom Breakpoints, the Shadow DOM etc) the void gets filled by a cacophony of ideas & suggestions, with often very little overlap.

This dramatic lack of consensus is a real hindrance. To borrow a phrase from the greatest Australian film ever made:

“I don’t know what the opposite of [a consensus] is, but you done the opposite.”

We done the opposite indeed, and we reinforce it every time we discuss new ideas as if the old ideas were horribly broken, when we’re really presenting incremental progress. So, while this article is most certainly about “new ideas”, let me state that however you’re doing CSS currently is totally fine 🙏 .

In particular, I think the combination of Sass, BEM & Gulp are the best choice for the majority of real-world projects right now. They allow you to get lots of stuff done quickly without leaving you with massive amounts of technical debt, with a huge body of examples and tutorials to learn from and plenty of knowledgeable people to ask.

My rating of Sass, BEM & Gulp: 💖💖💖

(It is well-known that pink sparkly hearts are the most powerful of all emoji)

And now it’s clear what we’re considering, let’s consider the future.

The Goal

There’s a lot of discussion at the moment about whether some or all of our styling code should be moved into JS. Projects like react-style, jss, radium & jsxstyle all offer competing ways to do that, but they’re coming at the problem from the wrong direction. I particularly like Keith Grant’s take on the matter:

Stop pretending the DOM and the JavaScript are separate concerns. Instead, separate concerns that are actually different: the dropdown menu is separate from the list of objects; the modal dialog box is separate from the page footer. Why on earth would you put these all in the same HTML document? The relationship between CSS and JavaScript is different. With HTML, a true separation of concerns between the markup and the corresponding component JS is impossible. With CSS, this separation is possible and even vital to clean code organization.

(edited for brevity)

There are three important questions to ask of potential styling workflows to judge how effective they are:

How quickly can you build & style a new component?

How much can you reuse styles when doing so?

How hard is it to change something to be visually unique later on?

This is what I aim for when I build anything. I want speed, reuse, but not unnecessary coupling, and modern CSS workflows (e.g. Sass & BEM) can nail all three. I see all the JS-styling approaches as falling at the first & second hurdles, so while I can appreciate that JS rendering gives us some new capabilities, it’s not worth taking a backwards step in terms of effectiveness. (For the record, my problem with all-encompassing CSS frameworks like Bootstrap are that they lead you to fall at the third hurdle.)

Sass & BEM do require a lot of convention & discipline to keep things getting out of hand, though. Maybe, then, the future of CSS is something that keeps the fluidity of CSS but automates the conventions. Something that takes the best of CSS & JS and combines them.

It turns out I’m not the only person who thinks this way, and recently we had enough people thinking the same way at the same time to affect change.

Dependencies across languages

From what I can tell, RequireJS was the first to do it but lately Webpack & JSPM have popularised the notion of require ing (or import ing because it’s 2015 already) a non-JS file that your JS depends on. For example:

import url from './logo.png' ; import './main-nav.css' ; export default class MainNav extends React . Component { render() { return < nav className = "MainNav" > < img src = {url} className = "MainNav_Logo" /> ... </ nav > } }

It’s up to the particular loader to decide what each import actually does, but by capturing a dependency from the JS to the asset files, we’ve gained a new capability. For example, we don’t have to hand-code exactly what the URL of the image file will be in production, we can just point to it locally. In fact, the loader responsible for the PNG file could pre-process the image by running it through an optimiser, then revision-stamp it, move it to an asset directory or a CDN, and return the URL. And indeed, that’s what Webpack’s image-loader and file-loader combine to do, and it works really well.

But what about the CSS in that previous example? We’re relying on convention that main-nav.css defines a class MainNav and MainNav_Logo , just the same as we used to rely on a convention for the URL to the PNG. We can do better.

What if a CSS file could export variables?

In the above example, there’s an obvious candidate – the classnames that a CSS file defines:

import url from './logo.png' ; import styles from './main-nav.css' ; export default class MainNav extends React . Component { render() { return < nav className = {styles.Nav} > < img src = {url} className = {styles.Logo} /> ... </ nav > } }

This approach has been popping up in a lot of places in the last few months. About ten weeks ago, Julian Viereck published a post on Medium entitled Modularise CSS the React Way. Three weeks later, Tobias Koppers (author of Webpack) added a concept of placeholders to his CSS Loader, and Guy Bedford (author of JSPM) suggested I look at something similar for JSPM. A month after that, Mark Dalgleish published The End of Global CSS, which showed a lot of people how much potential this approach had.

A New Syntax

Mark, Tobias & I started pursuing this idea further and developed it into a project called CSS Modules, but first we needed a standard way for the information to cross the CSS-JS boundary. All of my exploratory work had been specific to JSPM, all Mark’s and Tobias’ had been tied to Webpack. But there wasn’t anything inherently loader-specific about the idea, so we decided to combine our efforts. This is what we came up with:

ICSS: Interoperable CSS

ICSS is normal CSS with a couple of small additions. It’s similar to an Object File in compiled languages — the outputs of a compiler which can then be linked together to make a complete program. This is the first characteristic of ICSS:

✅ It is designed to be a compile target , not human-authored.

Compiled from what? Well, CSS Modules to start with, but it could be anything, the only requirement is:

✅ Each file is compiled separately then linked in the loader

Things like @import in Sass or CSS effectively concatenate the source code of multiple files so you can pass variables or mixins around, and many of the PostCSS plugins assume you’re doing something similar. In fact, most CSS processing is done globally because CSS has always been global.

JavaScript used to be like that as well. Before CommonJS, there wasn’t a standard for a.js to “require” b.js , they were both just running in the same global browser context and could make use of each other as needed. With the introduction of require() and module.exports , JavaScript became a programming language, with a proper system of dependencies and proper local scoping. ICSS is designed to do the same thing.

:export

The first task is to allow symbols to be exported from a file to JS or another CSS file. This is the :export pseudoselector block:

:export { Nav : _nav_nav_afd97dfs867 ; Logo : _nav_logo_97fd867fsfg ; } ._nav_nav_afd97dfs867 { } ._nav_logo_97fd867fsfg { }

The tokens are simply exported as a simple JS object:

import styles from './nav.css' ;

CSS Modules uses this to scope all class selectors by default, but under the hood it’s generating globally-unique classnames and using :export to provide them to JS.

:import

The other syntax addition is the :import pseudoselector. It allows a CSS file to declare a dependency on another and define which symbols it wants to import. It’s this latter capability that truly elevates what’s possible with CSS.

:import(" ./ utils .css ") { i__util_class_1: HorizontalNav; i__util_var_1: SharedUtilVar; }

Here, the path to the CSS file is provided as an argument. The loader will then go and fetch the file and link it with this one.

Next is the declaration of local temporary aliases and the exported symbols from the dependency that they represent. This block would match up to the following :export block of utils.css :

:export { HorizontalNav : _utils_horizontalnav_c7ab86431 ; SharedUtilVar : rgb ( 200 , 100 , 0 ) ; }

Here, we’re exporting a class name as HorizontalNav and a variable as SharedUtilVar but in reality they’re both just treated as simple strings.

Using imports

As a ICSS file is loaded and linked against its imports, the symbols get passed through and the :import block is deleted. Expanding on the above example:

:import(" ./ utils .css ") { i__util_class_1: HorizontalNav; i__util_var_1: SharedUtilVar; } :export { Nav : _nav_nav_afd97dfs867 i__util_class_1 ; } ._nav_nav_afd97dfs867 { color : i__util_var_1 ; }

After the :import is processed against the :export block in the previous section, the file becomes:

:export { Nav : _nav_nav_afd97dfs867 _utils_horizontalnav_c7ab86431 ; } ._nav_nav_afd97dfs867 { color : rgb ( 200 , 100 , 0 ) ; }

When this file is imported, the :export gets turned into a JS object and the remainder of the file is injected into the DOM by the loader.

High-level example

ICSS is designed as a compilation target, not to be coded by hand, so I thought I’d demonstrate how one of the features of CSS Modules is executed. Let’s look at composes , which is similar to Sass’ concept of @extend :

.outer { composes : flex-centered from "../utils.css" ; background : rgba ( 0 , 0 , 0 , 0.8 ) ; } .inner { composes : white-bg black-shadow from "./utils.css" ; border-radius : 4px ; }

This is saying that .outer includes all the styles from .flex-centered and .inner includes .white-bg and .black-shadow , where all the util classes are defined in utils.css :

.flex-centered { display : flex ; justify-content : center ; align-items : center ; } .white-bg { background-color : #eee ; } .black-shadow { box-shadow : 0 0 0 1px black, 0 0 8px - 2px rgba ( 0 , 0 , 0 , 0.8 ) ; }

These styles are used in JS by import ing them:

import styles from "./my-component.css" export default class MyComponent extends React.Component { render() { return <div className={styles.outer}> <div className={styles.inner}> /* content */ </div> </div> } }

The design goal for CSS Modules is to write something that looks global but is compiled to be localised. As with all pre-processors targeting ICSS, that compilation happens per-file. This is the compiled ICSS output:

:import(" ./ utils .css ") { i__util_class_1: flex-centered; i__util_class_2: white-bg; i__util_class_3: black-shadow; } :export { outer : _mycomponent_outer_ab24c761 i__util_class_1 ; inner : _mycomponent_inner_145bfed2 i__util_class_2 i__util_class_3 ; } ._mycomponent_outer_ab24c761 { background : rgba ( 0 , 0 , 0 , 0.8 ) ; } ._mycomponent_inner_145bfed2 { border-radius : 4px ; }

:export { flex-centered : _util_flexcentered_be5fd72ac ; white-bg : _util_whitebg_6dc31abb ; black-shadow : _util_blackshadow_9cd82af23 ; } ._util_flexcentered_be5fd72ac { display : flex ; justify-content : center ; align-items : center ; } ._util_whitebg_6dc31abb { background-color : #eee ; } ._util_blackshadow_9cd82af23 { box-shadow : 0 0 0 1px black, 0 0 8px - 2px rgba ( 0 , 0 , 0 , 0.8 ) ; }

When these files are loaded, the following JS object is returned:

import styles from "./my-component.css" // styles: { // outer: "_mycomponent_outer_ab24c761 _util_flexcentered_be5fd72ac" , // inner: "_mycomponent_inner_145bfed2 _util_whitebg_6dc31abb _util_blackshadow_9cd82af23" // }

This demonstrates a couple of benefits that CSS Modules provides:

Each file can be processed independently (enables parallel & incremental builds)

Styles can be reused by components by exporting multiple classes for a single component instead of trying to alter the CSS

All styles are global-safe, combining a human-readable part and a guaranteed-unique part in development. For production, these classes could be made far smaller while still ensuring uniqueness.

While CSS Modules is opinionated, ICSS is not. Which brings me to its final design characteristic:

✅ ICSS is designed to enable the capability of CSS to be loaded and linked together, not to make a judgement on the best way of doing so.

The Standard

By publishing a Interoperable CSS Standard, we’re hoping to unify the way we can treat CSS as a multi-file language, to then explore the impact that has on the authoring process. The three major loaders all support the format: Webpack’s css-loader, JSPM’s jspm-loader-css and Browserify’s css-modulesify.

If this capability proves to be as useful for the wider community as it has for us on the CSS Modules team, who knows where this leads? Maybe it can follow in the footsteps of CommonJS paving the way for the ES6 Module Spec & ASM.js becoming WebAssembly, and help shape the WhatWG Loader Standard? Maybe even run natively in the browser, one day? In the mean time, we can explore ideas and share our work wherever possible.

A world with Interoperable CSS in the browser.

Lionel Hutz wanted to write everything in JS.

If you have comments or suggestions, please send me an email or raise an issue on the ICSS repository. If you’re interested in CSS Modules, you can read more about it on its project page.

Happy interoperating, friends!