Implementing an html rendered that is compatible with all the css rules and it also implements all the extra workarounds that the current browser implement ( like handling illegal markup is not trivial) markup . I mean think about it, you hit invalid html markup and what do you do? You need to continue but now you need to check what Firefox or Chrome do for each case (missing end tags , P inside span, weird nesting of elements ) I would not want to implement a browser that is compatible with the current garbage.

There was also an article earlier this month or previous month from a Firefox developer about font/text rendering, in case you missed it rendering text in a way that works with all writing system is super hard.