[jstimpfle]

Page layout need not be complicated if you write it yourself in simple imperative code, for your own website.

But parsing the different standards of HTML and deal with invalid HTML in the "appropriate" way must be a lot of work. And implementing CSS to work with existing websites, and making it efficient, must be a PITA.

Much of that complexity was created in the name of discoverability. Given the amount of money that was spent on search and looking at the quality of search results nowadays, I'm not so sure.

In general the idea is to separate structure from style, and to allow developers to specify style more declaratively to enable them to make sites quickly. But I'm not positive that that worthwhile goal implies that the logic should be implemented in the browser. IMO it should be implemented in downloadable library code.