Use empty string for IDREF attribute reflection

empty-string-idref-attribute-reflection.md

Use empty string for IDREF attribute reflection

This is about attribute reflection for element references for things like ariaLabelledByElements.

Problem

Imagine we have a label an an input:

  <label id="foo">A label</label>
  <input id="myinput" />

And we want to set a relationship between both in JavaScript:

  myinput.ariaLabelledByElements = [ foo ];

Right now with the current spec text, the attribute aria-labelledby will be set to foo in the input element. That's known as sprouting:

  <label id="foo">A label</label>
  <input id="myinput" aria-labelledby="foo"/>

This has a problem if something changes later, as we can leave out of sync IDs values there in different situations. Examples:

• The label ID gets modified:

    foo.id = "bar";

  The HTML would be:

    <label id="bar">A label</label>
    <input id="myinput" aria-labelledby="foo"/>

  But if you check the relationship from JavaScript or in the a11y tree, you still get the proper label associated to the element. This would be very confusing if there's another element called foo on the tree, as the attribute would be somehow pointing to that other element, while the actual relationship set is pointing to the bar one.
• The label element is removed from the tree:

    document.removeChild(foo);

  In this case attribute would have a stale ID:

    <input id="myinput" aria-labelledby="foo"/>

Proposal

Set always the empty string, that means that a element reference has been set from JavaScript. And we never get into the case of having an out of sync ID. If needed people could query which elements have an empty attribute to know if they have an underlying relationship set.

In the previous example when we do the association we'll get:

  <label id="foo">A label</label>
  <input id="myinput" aria-labelledby=""/>

So whatever changes later regarding the label element has no bad effects.

Links

• Issues: whatwg/html#8306 & whatwg/html#8307
• PR: whatwg/html#8352
• Tests: web-platform-tests/wpt#36253