fix: Allow only-dlitem / only-listitem to have any hidden content

[WilcoFiers]

This change allows any hidden element in ul, ol and dl elements.

Closes #1021

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[jeeyyy]

Lgtm.

[dylanb]

I don't agree with the treatment of aria-hidden

To elaborate: aria-hidden="true" is intended to hide things from the assistive technology. It is not intended to override semantics. role="presentation" is intended to override semantics.

[dylanb]

hmmm, I don't agree with this case

[dylanb]

I disagree with this

[WilcoFiers]

@dylanb I don't understand your point. Assistive technologies treat elements with aria-hidden exactly the same way as they do elements with display: none. The following three lists look and behave exactly the same way as far as I know. That suggests to me that they should all pass.

<ul>
  <template is="dom-repeat">
    <li>{{text}}</li>
  </template>
  <li>Item 1</li>
  <li>Item 2</li>
</ul>

<ul>
  <dom-repeat style="display:none;">
    <li>{{text}}</li>
  </dom-repeat>
  <li>Item 1</li>
  <li>Item 2</li>
</ul>

<ul>
  <dom-repeat aria-hidden="true" style="position:absolute; top:-999em;">
    <li>{{text}}</li>
  </dom-repeat>
  <li>Item 1</li>
  <li>Item 2</li>
</ul>

[dylanb]

ok, I was mistaken in thinking this was a 4.1.1 rule and not a 1.3.1 rule