property string-set / content-list: attr(<identifier>)
Need to implement
string-set: <name> attr(<identifier>);
The string-set property contains one or more pairs, each consisting of an custom identifier (the name of the named string) followed by a content-list describing how to construct the value of the named string.
content-list expands to one or more of the following values, in any order.
content-list = [ <string> | <counter()> | <counters()> | <content()> | attr(<identifier>) ]+
The result we should get: attr-correct.pdf