Customized State Pseudo-Courses in Chrome

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There may be an growing variety of “customized” options on the internet platform. We have now customized properties (–my-property), customized components (<my-element>), and customized occasions (new CustomEvent(‘myEvent’)). At one level, we’d even get customized media queries (@media (–my-media)).

However that’s not all! You may need missed it as a result of it wasn’t talked about in Google’s “New in Chrome 90” article (to be honest, declarative shadow DOM stole the present on this launch), however Chrome simply added assist for yet one more “customized” function: customized state pseudo-classes (:–my-state).

Constructed-in states

Earlier than speaking about customized states, let’s take a fast have a look at the built-in states which can be outlined for built-in HTML components. The CSS Selectors module and the “Pseudo-classes” part of the HTML Customary specify quite a lot of pseudo-classes that can be utilized to match components in several states. The next pseudo-classes are all broadly supported in at present’s browsers:

Consumer motion

the mouse cursor hovers over the factor

the factor is being activated by the person

the factor has the main focus

the factor has or accommodates the main focus


the hyperlink has been visited by the person

the factor is focused by the web page URL’s fragment


the shape factor is disabled

the enter factor is displaying placeholder textual content

the checkbox or radio button is chosen

the shape factor’s worth is invalid

the enter factor’s worth is exterior the specificed vary

the enter factor has been autofilled by the browser


the customized factor has been registered

Observe: For brevity, some pseudo-classes have been omitted, and a few descriptions don’t point out each potential use-case.

Customized states

Like built-in components, customized components can have completely different states. An internet web page that makes use of a customized factor could wish to type these states. The customized factor may expose its states through CSS courses (class attribute) on its host factor, however that’s thought-about an anti-pattern.

Chrome now helps an API for including inner states to customized components. These customized states are uncovered to the outer web page through customized state pseudo-classes. For instance, a web page that makes use of a <live-score> factor can declare kinds for that factor’s customized –loading state.

live-score {
/* default kinds for this factor */

live-score:–loading {
/* kinds for when new content material is loading */

Let’s add a –checked state to a <labeled-checkbox> factor

The Customized State Pseudo Class specification accommodates a whole code instance, which I’ll use to clarify the API. The JavaScript portion of this function is positioned within the customized factor‘s class definition. Within the constructor, an “factor internals” object is created for the customized factor. Then, customized states will be set and unset on the interior states object.

Observe that the ElementInternals API ensures that the customized states are read-only to the skin. In different phrases, the outer web page can’t modify the customized factor’s inner states.

class LabeledCheckbox extends HTMLElement {
constructor() {

// 1. instantiate the factor’s “internals”
this._internals = this.attachInternals();

// (different code)

// 2. toggle a customized state
set checked(flag) {
if (flag) {
} else {

// (different code)

The net web page can now type the customized factor’s inner states through customized pseudo-classes of the identical identify. In our instance, the –checked state is uncovered through the :–checked pseudo-class.

labeled-checkbox {
/* kinds for the default state */

labeled-checkbox:–checked {
/* kinds for the –checked state */

Attempt the demo in Chrome

This function shouldn’t be (but) a typical

Browser distributors have been debating for the previous three years easy methods to expose the interior states of customized components through customized pseudo-classes. Google’s Customized State Pseudo Class specification stays an “unofficial draft” hosted by WICG. The function underwent a design overview on the W3C TAG and has been handed over to the CSS Working Group. In Chrome’s ”intent to ship” dialogue, Mounir Lamouri wrote this:

It appears like this function has good assist. It sounds that it could be arduous for internet builders to profit from it so long as it’s not broadly shipped, however hopefully Firefox and Safari will observe and implement it too. Somebody has to implement it first, and provided that there aren’t any foreseeable backward incompatible adjustments, it sounds protected to go first.

We now have to attend for the implementations in Firefox and Safari. The browser bugs have been filed (Mozilla #1588763 and WebKit #215911) however haven’t acquired a lot consideration but.

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