How many times have you look into webkit or gecko
source code to fix layout problems?
1. Adobe are themselves signaling that
HTML is the future by releasing HTML5 tools (Edge)
2. Adobe have abandoned mobile Flash development
3. Developer mind-share is firmly in the HTML camp.
If you think a layout engine is complex enough that
need to be open sourced, you are doing it wrong.
native app development (Visual Studio, XCode) layouts are much
simpler and more powerful
The problem with HTML and CSS is that they are either
document flow based or absolute position.
It's easier to do it right in native ways. There are much richer containers,
control groups and docking options.
Going back to the original point, I think that CSS for native layout may be an OK idea, CSS certainly has it share of strengths and weaknesses. One thing this approach has in its favor is a low barrier to entry for web developers which may help bridge the designer/developer divide.
It's been fun sparring with you :)
all the best,