I felt the same way. It's hard enough to remember the subtleties of CSS and which order the table tags go in. The header tag makes my head hurt(no pun intended).
To me, it's clear that they have spent more time making a rendering engine that is more compatible with CSS and Javascript. Every other popular browser(including Opera) has some serious Javascript and CSS rendering issues where it counts. From my development experience Firefox is the most developer friendly browser, by miles.