IBM actually had a research project back in 2000-2001 that worked this way, called "Sash": http://www.ibm.com/developerworks/library/it-0701art3/

I played with it a bit at the time and was amazed it never took off. Probably it was just too far ahead of its time; JavaScript was still stuck in its "oh, that's what you use to make icons dance on crappy web pages" bad-reputation era.