spec reading: semantics elements and styling

Hi,

The spec reads

> The default rendering of a semantics element is the default rendering of
its first child.

(http://www.w3.org/Math/draft-spec/chapter5.html#mixing.semantic.elements)

I'd take this to mean that (by default) the first child is rendered as if
it stood alone. But that seems problematic in an HTML5 context.

On the one hand, Firefox won't render the following mtable construction at
full width

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
  <semantics>
    <mtable width="100%" mathbackground="red">
      <mtr> <mtd><mi>x</mi></mtd> </mtr>
    </mtable>
    <annotation encoding="application/x-tex">x</annotation>
  </semantics>
</math>

On the other hand, I would expect (in HTML5) that styling the semantics
element (while poor practice) would affect the first child (and in fact it
does in Firefox).

So I'm wondering how to reconcile these two points of view (and what else I
might be missing here).

Best wishes,
Peter.