WordNet records no synonym and no antonym for markup. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the amount added to the cost to determine the asking price
detailed stylistic instructions for typesetting something that is to be printed; manual markup is usually written on the copy (e.g. underlining words that are to be set in italics)
Synonyms, antonyms and the broader and narrower relations for markup come from Princeton University’s WordNet. Full sources and licences.