Missing availability of aria-placeholder property with combobox role

[MarioBatusic]

Just as using the textbox role in the absence of a <input> element makes a <div> element with contenteditable an input for ATs, the same applies to the combobox role in the last spec version:

Encoded on a <div> or <span> element with contenteditable, this role also signals to ATs that it is an input with additional values offered for selection.

Since the HTML placeholder attribute is not available in this case, the aria-placeholder should be available on elements with the combobox role.

[scottaohara]

wonder if it would make sense to allow the attribute only in situations where the combobox actually allows text input? so, if a combobox has been provided an [aria-autocomplete](https://w3c.github.io/aria/#aria-autocomplete) attribute, then a placeholder would make sense. but if it's a select-only combobox, then maybe it shouldn't have a placeholder since it doesn't really have a purpose in that use case / people would be most likely to use it to state something like "choose a value"?

[jnurthen]

@MarioBatusic would you like to create a PR for this?

[scottaohara]

quickly testing with chromium per the comment in the WG meeting today that placeholder isn't respected on an element with
role=combobox

if the element with the explicit or implicit role=combobox is an input, the placeholder attribute will be exposed.

if the element with an explicit role=combobox is not an input (e.g., it's a button), then the placeholder is not exposed (though it is noted as existing in the a11y properties the dev tools displays)

<input list=d placeholder=foo aria-label=test>
<datalist id=d>
	<option>glarp</option>
</datalist>

<hr>

<button role=combobox aria-label=test aria-placeholder=foo aria-expanded=false>
	<span aria-hidden=true>boop</span>
</button>

<hr>

<input role=combobox placeholder=foo aria-expanded=false **aria-label=test>**