It won’t, it calls out to computed heading level.

But also I think we want slotted content to correctly compute heading level right? It would be unfortunate to have slotted content not derive its level from the slot and such.

I guess I'd like to discuss this more broadly with folks because I think we really want to use the flat tree. If we don't use the flat tree it'll mean slotted elements can only rely on the host element for offsets which essentially precludes the possibiltiy of making a compositional element which creates a heading heirarchy.

So like

<my-article>
  <h1 slot=title>The title</h1>
  <h1 slot=subtitle>The subtitle</h1> 
</my-article>

Something like this cannot be implemented, where the my-article component is able to slot various headings into the right positions and correctly build a heading heirarchy.

I'm somewhat surprised about lang but I guess there aren't many use cases where a page will switch language, but headings are substantially different to language because every page will have multiple heading levels.