ServoStyleSheets need ability for modification, which implies dirty flags and mInner cloning similar to the implementation in CSSStyleSheet.
Hm, I'd think that we already support mutation stylo stylesheets via our CSSOM stuff. Is the issue that we just don't expose this to C++ consumers over certain APIs? What are the consumers of those APIs?
I think the issue is that they can be modified, but we don't handle this correctly in the presence of style sheet inner sharing.
Ok, that makes sense. P1.