Related to #93 and #20.

While looking into shadow DOM and slots (#167 (comment)), I found an apparent disagreement between UAs.

Consider this DOM (Codepen):

<label id="the-label">
  <div>
    #shadow-root
      foo <slot aria-label="label"></slot> bar
    slotted
  </div>
</label>
<button aria-labelledby="the-label"> 

(Note the aria-label on the <slot>.)

What should the <button>'s accessible name be in this case? The UAs disagree:

Firefox and Safari ignore the aria-label, whereas Chrome treats it as significant.

The result is the same for default slotted content (see the same Codepen):

Per the HTML spec:

A slot element represents its assigned nodes, if any, and its contents otherwise.

Per this definition, it seems to me that the Firefox/Safari behavior is correct, and Chrome's is incorrect. (I.e. the slot's aria-label should be ignored; only its contents should be considered to "represent" it.)

Note this should also apply to aria-description, aria-labelledby, and aria-describedby. (I did not test these though.)