The <marquee> change is almost unique in that its effects are entirely negative — it makes Mozilla (trivially) worse in several respects, and better in none.

Initial thoughts: had a quick read of the bugnotes… seems like xhtml transitional pages will render in “strict mode” if served as text/xml or in “almost standards” as text/html. It’s a clean solution to squash a few of the barriers for adoption of Gecko. I like mozilla.org’s traditional militant stance on standards complience. The new mode is a standdown, and bad in the long-term.