Experience with HTML and CSS is mostly not transferable; it means you have some exposure to the requirements of syntax and the principle that computers do what you tell them, not what you want, but programming is much different. If you've used Javascript or PHP (or some other backend language) extensively in your web development, that would be more applicable knowledge.
Game development frequently uses a lot of pretty complex programming techniques, but explaining them when your programming experience is with web markup and the occasional scripting would be out of order. Similarly, I could list the frequently used libraries in game development (which all work pretty much the same anyway), but you wouldn't get far with them without a solid foundation in more general programming.
So you should start with some of the very useful tutorials that are out there for beginning programmers. The fundamental principles of programming are present in every programming language (though they're more visible in some than in others), and they transfer directly to other languages; and hobbyist game programming can be done in virtually any language (the professionals use C++ for performance reasons but C++ is kind of a pain; some languages' tools are better than others). So it doesn't really matter what language you start learning with.
There are some game development tools that don't require much programming knowledge at all, but they're necessarily limited, so I doubt you'll be satisfied with them for long.
So it depends on whether you prefer to A) learn just barely enough programming to scrape together a game that runs like shit, or B) put a lot of serious effort into learning how to do it the hard way before you can produce anything fun. These are by no means mutually exclusive, but I think you're going to want to do the latter sooner or later, so it might as well be sooner.