[logankeenan]

Do you mean the actual rendering of html/css when you say rendering differences? Or are you referring more to differences in JS support?

[yojo]

Pure html/css rendering. JS support could be addressed trivially in the build system by transpiling anything unsupported by the oldest likely target.

[ameliaquining]

Not all JS APIs are fully pollyfillable, especially ones that are part of the Web platform rather than ECMAScript. I dunno if it's a bigger problem overall than HTML/CSS, probably not, but it's not consistently trivial.