Need to implement string-set: <name> counter(<name-counter>);

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>) ]+
https://www.w3.org/TR/css-gcpm-3/#setting-named-strings-the-string-set-pro

HTML: counter.html

The result we should get: counter-correct.pdf