Parse a generic anchor-bearing element (tr, dt, section, li) into a ParsedSection.
W3C specs use these as named targets that don’t fit the dfn/heading pattern.
Parse an ecmarkup <emu-clause> or <emu-annex> element into a ParsedSection.
TC39 specs use these custom elements instead of standard headings.
The section ID is on the emu-clause, with an <h1> child containing the title.