experimentally eliminate comments from the grammar

[gimsieke]

…and see how they end up in the XML output. Both Gunther Rademacher and John Lumley have told me that then comments should just end up as text nodes in the XML output. John tried it with XPath 3.1, using the EBNF from https://bottlecaps.de/rex/
If it still doesn’t work with our CSS, we should investigate where the relevant differences are in the EBNF or in constructing the parser.