My own two cents: In order for something to be a programming language, it has to have three things: 1) a defined lexicon - the key words that make it do what it does; 2) defined control structures - this includes loop structures, logic control structures, even simple block structures; 3) temporary memory storage - in other words, variables.
This would include script languages, even T-SQL/PL-SQL, js, VB, BASIC, ASM, and tons more. The list probably includes more than it excludes.
As to whether HTML is a programming language... I'm of the opinion that it isn't. It's what its name says, a markup language. It also fails the third test - it doesn't allow for temporary memory storage; js does... but html by itself doesn't. It does have a defined structure, but it doesn't have control or looping constructs, so it half-fails the second test as well. Again, js includes these constructs, but html by itself doesn't have those capabilities. XAML is an even bigger grey area... at least html can stand on its own, xaml I don't think can... it requires more underneath it to really do anything... so I don't know... don't have much experience with it. It seems (like HTML) more of a UI definition language than an actual programming language; it can be paired with a programming language, but does that make it a programming language or an extension of one? shrug.

But that's just me and how I see it.

-tg