Do you mean the actual rendering of html/css when you say rendering differences? Or are you referring more to differences in JS support?
Pure html/css rendering. JS support could be addressed trivially in the build system by transpiling anything unsupported by the oldest likely target.
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.