The HTML 5 spec can be implemented from scratch, but my guess is it takes a dozen times more people than implementing a C++ compiler. My basis for this comment is the now-four-years-dormant project I worked on to implement just parsing HTML5 into a DOM tree, which wasn't even inspired by wanting to write my own browser. I was motivated more by having an interest in hacking text layout code, and the need for some sort of syntax for my test cases. Since the project had no commercial goal, it went down a tangent.

I'm impressed that a non-profit has funded two of these projects.