Browser vendors in the HTML Working Group
Standards groups are not your opportunity to order browser vendors to majorly shift their development strategy, neither Microsoft (as we have seen) nor anyone else. It is a place to work together and negotiate, yes. But if you ask browser vendors to do something they are fundamentally unwilling to do, like break compatibility, they will just walk away from the process. That’s how you end up with XHTML2.
For those of us who do not have the limitless development resources and cash pile of Microsoft, making HTML5 a completely separate mode is an unacceptably high cost. So it’s not going to happen. I know it’s lots of fun to make up something pure and clean and new, with all the ugly rough edges of compatibility filed off. But you can’t have both that and a spec that browsers will implement. Time to choose.
— Maciej Stachowiak (emphasis mine)