Equality for FontFace objects is tricky, and we don't get it right. We can't do a pointer equality on FontFace objects, because if the last JS reference to that object goes away, the object can be garbage collected. The underlying CSSFontFace object only has a weak reference to it's owning FontFace (because the FontFace has a strong reference to the underlying CSSFontFace object). However, we can't do pointer equality on CSSFontFace objects, because we may at any time cause the CSSFontSelector might rebuild (::buildStarted() / ::buildCompleted()) at any point, which will recreate all the CSSFontFace objects (and attempt to make the old FontFace objects adopt the new CSSFontFace objects). Instead, we must go deeper!
<rdar://problem/37092374>