Well, if you'll pardon the tautology, it's a language that's used for precisely expressing programs. Of course, that just shifts the question. What's a program?
It's a set of instructions for a computer to execute. Hopefully that's not controversial. But isn't `<input type="text">` an instruction to render a text input?
It is "controversial" because it's way too vague, which the pedant part should recognize (is a mouse click not an instruction for a computer to execute?), so of course you won't be able to differentiate at this level.
If anything, I made it too specific by saying computer.
> is a mouse click not an instruction for a computer to execute?
If an SOP document—another example of a program—says to click a button on the screen, then of course that action is part of the instructions for a program. No computer needed, even; the instruction could be to stick my thumb up my nose, for all it matters.
