dongryphon/focus-ring

Polyfill for `:focus-ring`

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

Demo

Rationale

The status quo, :focus, is quite problematic:

• Many developers disable the default focus ring in their CSS styles, others attempt to style it in concert with their design. The former often seems to be a result of finding the default focus ring both aesthetically unpleasant and confusing to users when applied after a mouse or touch event and introduces accessibility problems. The latter inevitably creates considerably more of the kind of problem that the former was trying to solve.
• Some native elements in some browsers, notably <button> in Chrome, have a "magic" focus style which does not apply unless focus was received via a keyboard interaction.

To deal with this:

• It seems evident that a visual indication of what has focus is only interesting to a user who is using the keyboard to interact with the page. A user using any kind of pointing device would only be interested in what is in focus if they were just about to use the keyboard - otherwise, it is irrelevant and potentially confusing.
• Thus, if we only show the focus ring when relevant, we can avoid user confusion and avoid creating incentives for developers to disable it.
• A mechanism for exposing focus ring styles only when the keyboard is the user's current input modality gives us this opportunity.

API Proposal

/* override UA stylesheet if necessary */
:focus {
  outline: none;
}

/* establish desired focus ring appearance for appropriate input modalities */
:focus-ring {
  outline: 2px solid blue;
}

:focusring matches native elements that are

1. focussed; and
2. would display a focus ring if only UA styles applied

Additionally, :focusring matches non-native elements as if they were native button elements.

Example heuristic

The heuristic used to decide the current modality should not be defined normatively. An example heuristic is to update modality on each style recalc: if the most recent user interaction was via the keyboard; and less than 100ms has elapsed since the last input event; then the modality is keyboard. Otherwise, the modality is not keyboard.

Implementation Prototype

The tiny focus-ring.js provides a prototype intended to achieve the goals we are proposing with technology that exists today in order for developers to be able to try it out, understand it and provide feedback. It simply sets a .focus-ring class on the active element if the script determines that the keyboard is being used. This attribute is removed on any blur event. This allows authors to write rules which show a focus style only when it would be relevant to the user. Note that the prototype does not match the proposed API - it is intended to give developers a feel for the model rather than to provide a high-fidelity polyfill.

We suggest that users selectively disable the default focus style by selecting for the case when .focus-ring is not applied:

:focus:not(.focus-ring) {
    outline: none;
}

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

How it works

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

• a focus event immediately following a keydown event
• 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.