It's probably true that it's difficult to decide what should be removed without breaking the browsing experience too much. Still, I bet that the engine still contains lot of code for legacy stuff from the era when we wrote HTML such as <table cellpadding="0"> etc.

The real problem of course is that the web specs itself have become too complex and expensive to implement. But a stripped-down version of browser could point out why we need to reboot the whole technology. It would be a long shot, I admit.