<Grid>
<TextBlock VerticalAlignment="Center"/>
<\Grid> <Grid>
<TextBlock VerticalAlignment="Center"/>
<\Grid>It's not that kind of "responsive design" we know from HTML. The startmenu only snaps to at least three sizes (vertical) and doing so with little UX feedback animation. It could be done better even with XAML (and it would be trivial with CSS3), but it was probably a design choice - it's something for Raymond Chen to write about in ten years.
(And changing back from tablet-mode startmenu to desktop-mode startmenu is a bit hidden in the icon next to the clock and takes some time to locate the setting.)
Given the choice, I'd choose XAML over HTML/CSS for app layout/design in a heartbeat.
I think it all goes back to what each language was designed for and people hacking things into HTML/CSS to get it do things it wasn't designed for.