Comment 1 for bug 1451937

The fundamental problem here is that CSS doesn't offer any separation between a "public API" and private implementation details. And because they're "cascading", a basic change can end up affecting all sorts of elements.

While we could fix this particular problem, avoiding this class of problems is more difficult. Some ideas:
1) Don't ship the toolkit on the device. Make developers include a known-good version in their app.
2) Aggressive versioning. Every time you make a change that could break apps, bump a version number. But this would required app authors to keep updating their apps. Also gives a bunch of versions to potentially backport fixes to. I don't see that this is any better than (1).
3) Publish a supported implementation "API" -- particular layout and display techniques that will be guaranteed to work. Developers can either stick with those and use the system toolkit or implement more complicated layouts and ship a known-good version of the toolkit in their packages.