Don't enable polyfill if browser supports ":focus-visible"

[kutensky]

Currently Chrome with experimental flag enabled supports ":focus-visible" polyfill. It would be nice if polyfill could be enabled only if current browser doesn' support ":focus-visible". To do that, I propose to check pseudo-class support using this method:

supportsPseudo = function (pseudoClass) {
    // Get the document stylesheet
    var ss = document.styleSheets[0];

    // Create a stylesheet if one doesn't exist
    if (!ss) {
        var el = document.createElement('style');
        document.head.appendChild(el);
        ss = document.styleSheets[0];
        document.head.removeChild(el);
    }

    // Test the pseudo-class by trying to style with it
    var testPseudo = function () {
        try {
            if (!(/^:/).test(pseudoClass)) {
                pseudoClass = ':' + pseudoClass;
            }
            ss.insertRule('html' + pseudoClass + '{}', 0);
            ss.deleteRule(0);
            return true;
        } catch(e) {
            return false;
        }
    };

    // Run the test
    return testPseudo();
};

And then in polyfill modify code on the line 306 with:

if (typeof document !== 'undefined' && !supportsPseudo("focus-visible")) {
    // Apply the polyfill to the global document, so that no JavaScript
    // coordination is required to use the polyfill in the top-level document:
    applyFocusVisiblePolyfill(document);
}

After that we will be able to write css rules for both native ":focus-visible" and polyfill version:

:focus:not(:focus-visible) {
    outline: 0;
}
.js-focus-visible :focus:not(.focus-visible) {
    outline: 0;
}

[emilio]

Drive by, but that's a really really expensive way to test for a pseudo-element. try { document.querySelector(":focus-visible"); return true; } catch { return false } should be much faster.

[Justineo]

I’m afraid this requires double your style declarations for .focus-visible to make it work with or without the “polyfill” (technically this project isn’t a real polyfill).

[kutensky]

I’m afraid this requires double your style declarations for .focus-visible to make it work with or without the “polyfill” (technically this project isn’t a real polyfill).

Yeap, that will require a double style declaration. But on the other hand, when browsers start to support his feature (Chrome is going to start supporting it from v.86), no js run will be needed. Currently, polyfill does a lot of background work that isn't good for performance.

[robdodson]

Very good points from everyone.

It sounds like if we land this then we should use @emilio's implementation, but also land a note in the README which explains the need to double up your style declarations. We could also add a note that if folks would prefer, they can use the postcss plugin (https://github.com/csstools/postcss-focus-visible) which I think will let them write their CSS using :focus-visible and then it'll do the right thing depending on their browser support matrix. I'm not a post-css expert but I think that's how it works :)

[ryuran]

Related to #244

This condition could help

if (!window.CSS?.supports?.("selector(:focus-visible)")) { 
  // Apply the polyfill
}