I think your second argument also applies to the first no? Any technology that is implemented in a major browser, be it JS, Webkit, SQLite has incentives to port it to other platforms. Web developers don't expect 100% compatibility, they are so used to things behaving differently and broken across browsers that it's actually surprising if something just works from time to time.

If anybody was expecting 100% compatibility all the time, we wouldn't get any new standards, and would all use chromium.