The definition of "rendered legend" should use the box tree, or at least the flat tree.

[emilio]

That is how browsers (at least WebKit / Gecko / Blink) implement it, and it makes sense.

If it's defined using the flat tree it should account for display: contents ancestors (skipping them as appropriate).

[annevk]

Can you give examples that demonstrate the difference in definition? (The current definition already pokes at the box tree afaict.)

[emilio]

Trivial example would be:

<fieldset>
  <div style="display: contents">
    <legend>Legend</legend>
  </div>
</fieldset>

You could construct another with Shadow DOM adding a <slot> under a <fieldset> and slotting a <legend> inside.

https://html.spec.whatwg.org/#rendered-legend says:

If the fieldset element has a child that matches the conditions in the list below

Where child is the light tree child concept (https://dom.spec.whatwg.org/#concept-tree-child), which is the bit I expect to change with this issue.

[annevk]

I guess for legend this is reasonable as it cannot be otherwise observed, but we should be careful not to change all child-based lookup like this (e.g., when searching for option elements or some such).

[annevk]

Not entirely sure what terminology to use though as CSS doesn't really define the box model in sufficient detail. Would "first child box whose corresponding element is a legend element" work? Assuming we also keep the other conditions already in the standard.

[emilio]

I think that would match what's implemented in both blink and Gecko at least.

[emilio]

(And WebKit too, just checked).

May be worth to check edge in case it does something different, but I can't do that easily.

[zcorpan]

You could construct another with Shadow DOM adding a <slot> under a <fieldset> and slotting a <legend> inside.

Like this?

https://software.hixie.ch/utilities/js/live-dom-viewer/saved/6080

This seems to work in WebKit/Chromium/Gecko. (In EdgeHTML 17, customElements is not defined.)

[zcorpan]

To confirm, the wording suggested above ("first child box whose corresponding element is a legend element") would include the flat tree case?

[emilio]

Yes, it would. Elements outside of the flat tree don't generate boxes (or have styles for that matter).