Fortunately, we explicitly disallow curly brackets in element hiding rules, note `[^{}]` in the [regexp](https://hg.adblockplus.org/adblockplus/file/2d52467436ed/lib/filterClasses.js#l77). Any other reason why we need both `.querySelector()` and `.insertRule()`?

I filed http://crbug.com/380033 on the `.querySelector()` issue, it is clearly a validation bug (not checking parser state correctly).