Definition

I would say that something is programming language (PL) if:

• 1: there is a machine that can do some actions (computations) based on the text written in the language (program)
• 2: or this language can be mechanically translated (compiled) to a programming language

Now let's see if we can tell based on this rule if something a PL or not.

• Machine code? Yes, it can be interpreted by CPU
• Assembly? Yes, it can be compiled to machine code.
• C? Yes, it can be compiled to an assembly.
• JavaScript? Yes, it can be interpreted by nodejs.
• Postscript? Yes, it can be interpreted by Ghostcript.
• HTML? Yes, it can be interpreted by a browser.

Wait a second ... is HTML a programming language?

Yes according to my definition of PL (if you disagree provide your definition). There is a grammar, which specifies how to parse it. There is an interpreter - in response to instruction in the program it renders different things on the screen.

But is it Turing complete?

Irrelevant question.

• Brainfuck is Turing complete, but do you want to program with it?
• Agda and Coq are not Turing complete, but nobody seems to doubt that those are programming languages

1. Beware of the Turing tar-pit in which everything is possible but nothing of interest is easy.

-- Alan Perlis

Turing tarpit examples: Brainfuck, C++ templates.

Related: "Pacman complete" (a term coined by Edwin Brady?) - how is it easy to implement Pacman game in the programming language (as an alternative to Turing completeness).

General-purpose vs special-purpose

Or maybe you want to ask: is it a general-purpose PL? No, it's not - it has a special purpose to denote markup for hypertext. Postscript is Turing complete, but it is used for the same purpose as HTML - to denote markup.

Another name for special-purpose PL is a domain-specific language (DSL).

Declarative vs imperative

Declarative programming is when you write your code in such a way that it describes what you want to do, and not how you want to do it.

From my point of view, this is a very "sketchy" definition. For example, in CSS (before flexbox) to center div you would do something like this:

.container {
  ...
  position: relative;
}
.child {
  width: 50px;
  /* Center vertically */
  position: absolute;
  top: 50%;
  margin-top: -25px; /* half this element's height */
}

Is it fair to say that I'm expressing "what" instead of "how"? Effectively I say move 50% from top and half of height back.

Let's continue with the quiz

• XML? In general case no, in special cases yes.
  • XHTML? Yes
  • Apache ant? Yes
  • SVG? Yes

XML itself has no meaning, it describes the structure. But as soon as you declare an application that can interpret XML you "define a meaning" for it.

PS

If you don't know how compilers work, then you don't know how computers work.

-- Steve Yegge

I would change it a bit: "if you don't know how compilers and interpreters...".