A lot of the JS problems arise from developers building against v8. I primarily develop with Firefox and do the same the other way: wrote a demo last year that ran at 60fps in Firefox, but <15fps when I tried it in Chrome. Turned out that out-of-index array accesses drop v8 out of a performance optimisation, but had I developed against it in the first place I might have abandoned the whole approach as lacking performance instead of just realising it was a quirk of a particular engine. Incidentally rewriting it to guard against out-of-array accesses only got Chrome up to 50fps so it was still down on Firefox. Maybe other assumptions of mine?