As far as I can tell from the description of AdaptiveMetricsContext (from the new Longhorn XAML specs) that formatting engine misses one of the qualites of HTML/CSS, namely that the styling information can be kept almost completely separate from the content. When the logical model of a page is established (via e.g. nested div tags) no other styling information goes into a page. Only content.
This is a good separation of concerns, and seems to be missing from the new MS format. Since the new MS formats are all XML the separation may be implemented anyway using e.g. xslt's but it is not built in to the format as far as I can tell.