Created attachment 8902754 [details] testcase.html In the attached testcase, the <symbol> has both the attribute fill="none" and the CSS fill:red. However, the red does not apply to <use> instances, when it should (all other browsers behave this way). As such the <use> instances end up with no fill, rather than a red fill, unlike in other browsers (Edge, Chrome, Safari). The reason seems to be that since the CSS is applied specifically to the original symbol's path, it is not copied over to the instances as well. If I add in an old-school CSS hack (as in the comment in the attached test-case), then Firefox will also propagate the fill accordingly. I'm unsure if this is an edge-case missed in bug 265894 or not, but I thought I'd mention it just in case.
I don't think this is valid per https://www.w3.org/TR/SVG2/struct.html#UseStyleInheritance and I don't think any other UA has implemented this part of SVG 2 yet either. Someone has to be first and this time it's us.