[css-typed-om] Spell out the obvious

[j9t]

The CSS Typed OM [1] misses to spell out what it’s about, the Cascading Style Sheets Object Model. I suggest the abbreviations get expanded at least once. (Even though we are familiar with the matter, others who may run into this may not; the meaning of “OM” may be unclear even to people from our field.)

[1] https://www.w3.org/TR/css-typed-om-1/