Good arguments here so far.
Only addition I see needing to be made is:
If CSS and HTML aren't programming languages then what are they?
Seems like tweeting guy Marion has the ball in his court: if they aren't the thing that we think that they are, what are they?
If Marian wants to attack an essential definition, he ought to suggest an alternative. Epistemology and linguistic understanding aren't democratic, and if any of us tried to explain CSS and HTML to inexperienced person, we'd probably call them "programming languages". This is pretty clearly someone trying to agitate and demean with a tweet rather than a real contribution to understanding.
...or maybe there is a cool word for the category of thing that CSS and HTML are that I don't know?
That's a great point, and one I haven't thought of! For people who are adamant that they aren't programming languages, I've never seen them present a note on what they are.
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