How far are you planning to go in your support for css properties ? css animations/transitions ? Positionning (top, left, right) ? There are a lot of similarities, but by going very far you may end up rewriting a lot of the storyboard xml in a css format...
Then maybe later on, you'll want a WYSIWYG css editor on top of it, and i'm a bit wondering whether you won't have ended up rewriting absolutely everything.<p>From my personal opinion, CSS is nice the first 10 minutes, or whenever you only have a few element to style, but styling a complete app along with animations between states makes you really want to stay away from it as far as possible for mobile apps (and especially ios where resolutions are quite standard)...