HTML5 is not a language, it's a widely abused term that is used in reference of many new-age technologies that are appearing in web development lately.
If you want to learn how to hack and crack codes, then HTML isn't for you. It's hardly a programming language, it's just a very simple markup language, although it is good to learn first so that you can get used to writing many lines of code and so on.
My personal recommendation is Visual Basic.Net. It's an extremely easy programming language developed by Microsoft, and the rules of writing it are very relaxed. Some language demand a semi-colon at the end of each line, or have other specific guidelines, where Visual Basic.Net isn't very strict, therefore I find it easier to learn since you're aren't worried about missing a period or semi-colon somewhere.
Also, Visual Basic.Net has a ton of built in code that is extremely easy to use for some things you would probably enjoy, such as interacting with a website(since you seem to have an interest in messing with other peoples programs, like I do). The first real program I built with VB.Net solved a basic captcha and then automated the game play of a web-based game I enjoyed... so you may enjoy it also.
Overall, it's hard to tell you what language to pick. There's many of them, and each has it benefits or downsides, and it really depends on what person you're speaking to. It also depends on what you want to accomplish. Such as HTML for websites, or VB.Net for PC programs. Just dabble and find what you like. I've personally programmed in Java, C++, C#, JavaScript, PHP, and many other, and still prefer VB.Net.