There's a W3C rule, that each browser needs their own implementation. If everybody uses the same code, it's considered a Monoculture security risk [1].
1. https://www.schneier.com/blog/archives/2010/12/software_mono...
1. https://www.schneier.com/blog/archives/2010/12/software_mono...
Somebody posted it: https://news.ycombinator.com/item?id=10199163