Spans with "smallcaps" as the first class are converted to *SmallCaps*
elements. While previously no other classes or attributes were allowed,
additional classes, attributes, and an identifier are not permitted and
kept in a *SmallCaps* wrapping *Span* element.
The same change is applied to underline spans, where the first class
must be either "ul" or "underline".
Closes: #4102
