I hit this assertion and according to the local variables on the stack the SetAttr in question has a name of "crossorigin" and an empty value.
Created attachment 553748 [details]
WinDbg stack trace
Looks like they also set it to the empty string in script too...
Created attachment 554988 [details]
nsAttrValue::SetMiscAtomOrString has this code:
PRUint32 len = aValue->Length();
// We're allowing eCSSStyleRule attributes to store empty strings as it
// can be beneficial to store an empty style attribute as a parsed rule.
// Add other types as needed.
NS_ASSERTION(len || Type() == eCSSStyleRule, "Empty string?");
Jonas, sounds like we should add enumerated types here, right?
Created attachment 555477 [details] [diff] [review]
Allow empty string values to be associtated to enumerated attributes.
Something in this push caused orange, so backed out of inbound:
The orange was something else. Pushed http://hg.mozilla.org/integration/mozilla-inbound/rev/aa18f8632882