Based on the proposed CSS :focus-visible pseudo-selector, this prototype adds a focus-visible class to the focused element, in situations in which the :focus-visible pseudo-selector should match.

Details

Polyfill

Installation

npm install --save focus-visible

We recommend only using versions of the polyfill that have been published to npm, rather than cloning the repo and using the source directly. This helps ensure the version you're using is stable and thoroughly tested.

If you do want to build from source, make sure you clone the latest tag!

Usage

1. Add the script to your page

    ...
    <script src="/node_modules/focus-visible/dist/focus-visible.min.js"></script>
  </body>
</html>

2. Update your CSS

We suggest that users selectively disable the default focus style by selecting for the case when the polyfill is loaded and .focus-visible is not applied to the element:

/*
  This will hide the focus indicator if the element receives focus via the mouse,
  but it will still show up on keyboard focus.
*/
.js-focus-visible :focus:not(.focus-visible) {
  outline: none;
}

If there are elements which should always have a focus ring shown, authors may explicitly add the focus-visible class. If explicitly added, it will not be removed on blur.

Alternatively, if you're using a framework which overwrites your classes (#179), you can rely on the data-focus-visible-added attribute.

.js-focus-visible :focus:not([data-focus-visible-added]) {
  outline: none;
}

How it works

The script uses two heuristics to determine whether the keyboard is being used:

a focus event immediately following a keydown event where the key pressed was either Tab, Shift + Tab, or an arrow key.
focus moves into an element which requires keyboard interaction, such as a text field
TODO: ideally, we also trigger keyboard modality following a keyboard event which activates an element or causes a mutation; this still needs to be implemented.

Dependencies

If you want to use :focus-visible with an older browser you'll need to include an additional polyfill for Element.prototype.classList.

In accordance with the W3C's new polyfill guidance, the :focus-visible polyfill does not bundle other polyfills.

You can use a service like Polyfill.io to download only the polyfills needed by the current browser. Just add the following line to the start of your page:

<script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=Element.prototype.classList"></script>

Shadow DOM

It could be very expensive to apply this polyfill automatically to every shadow root that is created in a given document, so the polyfill ignores shadow roots by default. If you are using Shadow DOM in a component, it is possible to apply this polyfill imperatively to the component's shadow root:

// Check for the polyfill:
if (window.applyFocusVisiblePolyfill != null) {
  window.applyFocusVisiblePolyfill(myComponent.shadowRoot);
}

Lazy-loading

When this polyfill is lazy-loaded, and you are applying the polyfill to a shadow root with JavaScript, it is important to know when the polyfill has become available before trying to use it.

In order to act at the right time, you can observe the global focus-visible-polyfill-ready event:

window.addEventListener('focus-visible-polyfill-ready',
    () => window.applyFocusVisiblePolyfill(myComponent.shadowRoot),
    { once:  true });

Important: this event is only intended to support late application of the polyfill in lazy-loading use cases. Do not write code that depends on the event firing, as it is timing dependent and only fired once. If you plan to lazy-load the polyfill, it is recommended that you check for it synchronously (see example above under "Shadow DOM") and listen for the event only if the polyfill isn't available yet.

Backwards compatibility

Until all browsers ship :focus-visible developers will need to use it defensively to avoid accidentally removing focus styles in legacy browsers. This is easy to do with the polyfill.

/*
  This will hide the focus indicator if the element receives focus via the mouse,
  but it will still show up on keyboard focus.
*/
.js-focus-visible :focus:not(.focus-visible) {
  outline: none;
}

/*
  Optionally: Define a strong focus indicator for keyboard focus.
  If you choose to skip this step then the browser's default focus
  indicator will be displayed instead.
*/
.js-focus-visible .focus-visible {
  …
}

As explained by the Paciello Group, developers who don't use the polyfill can still defensively rely on :focus-visible using the following snippet:

/*
  Provide basic, default focus styles.
*/
button:focus {
  …
}

/*
  Remove default focus styles for mouse users ONLY if
  :focus-visible is supported on this platform.
*/
button:focus:not(:focus-visible) {
  …
}

/*
  Optionally: If :focus-visible is supported on this
  platform, provide enhanced focus styles for keyboard
  focus.
*/
button:focus-visible {
  …
}

In the future, when all browsers support :focus-visible, the snippets above will be unnecessary. But until that time it's important to be mindful when you use :focus-visible and to ensure you always have a fallback strategy.

Big Thanks

Cross-browser Testing Platform and Open Source <3 Provided by Sauce Labs

MarcusMorba/focus-visible