fix aria-hidden text for visibility: visible;

[jnurthen]

aria-hidden states

User agents determine an element's hidden status based on whether it is rendered, and the rendering is usually controlled by CSS. For example, an element whose display property is set to none is not rendered. An element is considered hidden if it, or any of its ancestors are not rendered or have their aria-hidden attribute value set to true.

This is incorrect - visibility:hidden is rendered per HTML's definition ( https://html.spec.whatwg.org/multipage/rendering.html#being-rendered ) but is hidden, - and children of visibility:hidden can be set to visible and be rendered AND visible - so clearly not hidden.

[chlane]

I would like to work on this one. It seems we need to also explain that rendering and visibility are separate concepts.

[chlane]

@jnurthen since rendering doesn't have anything to do with hidden status can we just say " An element is considered hidden if it, or any of its ancestors have their aria-hidden attribute value set to true"?

[Jym77]

There seems to be a similar problem (not taking into account the fact that visibility: hidden can be changed) in tree exclusion:

The following elements are not exposed via the accessibility API and user agents MUST NOT include them in the accessibility tree:

• Elements, including their descendent elements, that have host language semantics specifying that the element is not displayed, such as CSS display:none, visibility:hidden, or the HTML hidden attribute.

In

<div style="visibility: hidden">
  Hello
  <div style="visibility: visible">World</div>
</div>

The outer <div> triggers that condition, which state that it must be excluded from the accessibility tee, including its descendants, but the inner <div>, which is a descendant, should be included instead 🤔

(see this discussion in W3C slack)