It's a terrible idea to let authors override the editability of UA shadow DOM via -webkit-user-modify. In fact, we already have a bunch of -webkit-user-modify: read-only !important rules in html.css. In order to resolve the bug 129034, completely disallow them in all cases in the engine itself. Heck, we probably want to disallow -webkit-user-modify anywhere in the shadow DOM since it's a legacy CSS property that's hurting the performance for us.
Created attachment 224843 [details] Patch
Comment on attachment 224843 [details] Patch r=me
Comment on attachment 224843 [details] Patch Clearing flags on attachment: 224843 Committed r164501: <http://trac.webkit.org/changeset/164501>
All reviewed patches have been landed. Closing bug.