> persuading Web developers to use it is where the real problem lies, and that is immensely difficult and no-one has any good ideas for how to do it
Let the different subsets compete. You'd have multiple different simple browser that each support a different subset. Developers would have to choose which one they agree with and make their sites compatible. They would all render properly in full browsers. You can find the sweet spot over time.
We lived through browser fragmentation before, with IE support vs other web standards. It just meant that developers had to make a version for everything except for big critical things like banks and .gov will be 'ie-only' (or maybe 'chrome-only', now).
It's way better to have one jankier universal platform with a few good implementations.
Let the different subsets compete. You'd have multiple different simple browser that each support a different subset. Developers would have to choose which one they agree with and make their sites compatible. They would all render properly in full browsers. You can find the sweet spot over time.