It's an opinion, take it with a grain of salt.
(BTW, I dislike MW because it takes all nuance out of the language and dumbs it down to an inexpressive mess.)
Obsolete is a verb, but it is a poor choice. A better
choice would be obsolesce, as in: "LESS.js will
obsolesce CSS"
"Obsolete" is the correct word because it's a transitive verb (eg, [subject] obsoletes [object]), while obsolesce is an intransitive verb (eg, [subject] obsolesces).If you take a gander at WordNet or even OneLook (searches multiple dictionaries), you'll find that the majority list obsolete only as an adjective, including Webster's 1828.
However, I concur that obsolesce would have been a better choice.
Sounds strange to the modern ear though.
(BTW, I used to be a grammar nazi years ago.)