I may sound strange but for me it was the other way around and HTML was way harder to learn than new programming languages. I learned programming long ago at a time there were no HTML around and grasped basic understanding of a few constructs (conditions, loop, variables, etc). These I found again when learning new languages, so learning new languages became easier and easier.
When I learnt HTML I saw no logic in it. Once you have learn the minimal syntax, all is about keeping in mind a bunch of arbitrary tags and obscure options (and whose behavior change from one browser to another).
I understand that is not the case for most people, probably because HTML is more like a static description of something.
Well, that is not the whole truth. Learning new languages became hard again for me at each change of programming paradigm, say from procedural like C to Object Oriented like Java, to functional like Haskell.
What I'm really saying is that I believe that any change of paradigm (basic assumptions of the domain) is hard. HTML and programming language are really different paradigm (programming arguably more complex as it's about describing changes and HTML describing state). When you go from one paradigm to another you have to learn some basic thruth again and it's hard.