问题:

There is a hack to make work :focus pseudoclass for div elements: adding to div tabindex. Like this:

.testFocus:focus{
    background: red;
}

<div class="testFocus" tabindex="0">awesomeDiv</div>

Is this behavior specified somewhere in W3C documents (where?) or is it just a non-documented hack?

"This behavior" consists of:

div element is not focusable by default.
div element with tabindex is focusable.

回答1:

tabindex is one of the global attributes. This means it can be specified on all HTML elements.

0 is a valid value (see "If the value is a zero" under the definition of tabindex).

So your HTML is fine.

回答2:

tabindex will work on the following elements in HTML5. https://www.w3.org/TR/html5/editing.html#sequential-focus-navigation-and-the-tabindex-attribute

a elements that have an href attribute

link elements that have an href attribute

button elements

input elements whose type attribute are not in the Hidden state

select elements

textarea elements