Crossword clue: a set of tags and rules (conforming to sgml) for using them in developing hypertext documents

The most common answer is html (4 letters).

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

Possible answers

html 4 letters wn-def

a set of tags and rules (conforming to SGML) for using them in developing hypertext documents