AX: Implement CSS -webkit-alt property
https://bugs.webkit.org/show_bug.cgi?id=120188

This is blocking 20+ bugs on one of our higher profile content sites and we’d 
like to start work on it. To clarify, the problem is that with CSS generated 
content in pseudo-elements like this:

        .expandable::before { content: "\25BA"; /* a.k.a. ► */ }

…there is no way to prevent VoiceOver from speaking the literal character 
description, “right pointing black pointer.” If this were an actual DOM node, 
we could hang some ARIA attributes on it, but there is no node for 
pseudo-elements, so the property has to be defined in CSS.

The CSS Working Group discussion indicates the editors (Tab from Google, and 
Elika from Mozilla) are generally on board with the concept of accessible text 
alternatives for CSS generated content and Tab suggested the property name. It 
is not in a CSS4 draft yet, but some usage examples are below.

Rendering a decorative disclosure triangle on a "collapsed” ARIA treeitem:

        [aria-expanded="false”]::before {
                content: "\25BA"; /* a.k.a. ► , literally “right pointing black 
pointer” */
                alt: ""; /* aria-expanded="false" already in DOM, so this 
pseudo-element is decorative */
        }

And this, where a style indicates new content, screen readers can speak “New” 
instead of “shadowed white star”:

        [data-new]::after {
                content: "\2730”; /* a.k.a. ✰ , literally “shadowed white star” 
 */
                alt: attr(data-new); /* allows for localized content from the 
DOM, e.g. @data-new="New!" */
        }

Any questions or concerns?

Thanks,
James


PS. Related to this one is bug 122138, where the “alt” can be specified inline 
after url() image content:

AX: WebKit does not expose text alternative of CSS generated image content
https://bugs.webkit.org/show_bug.cgi?id=122138

        .new::after {
                content: url(./star.png), "New!";
        }


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