(Un-assigning - I assume this was the unintended default from creating a subtask based on the parent).

This bug is similar to T151972: CSSMin should not convert urls for filters. However in this case there is one notable exception. The url values in filter always refer to a relative url, which means hash values will refer to an element on the page. This is intentionally not supported by ResourceLoader, as explained in T151972#2897445..

However, in the case of behaviour the value can be one of three things, as shown by http://stackoverflow.com/a/26339480/319266:

eirenaios at Stack Overflow:

• url(sLocation): [..] where sLocation is an absolute or relative URL.
• url(#objID): [..] where objID is the ID [of] an object tag [on the page].
• url(#default#behaviorName): [..] The application's default behavior, identified by its behaviorName.

This last one does not refer to an element on the page, rather it refers to special reserved words that just happen to look like a hash fragment.