XAML is far easier for app layout than HTML/CSS. It's very easy to do responsive design. We do have a full screen layout for those that want it, and it's what you get in Tablet Mode.
I meant resizing the startmenu with the mouse by grabbing the border and moving the mouse.
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.)