HTML and CSS are the basics of Frontend, almost every single technology developed later on to make Frontend easier, still uses the basics from HTML and CSS, so learning the basics of HTML and CSS are a given for Frontend developers / Frontend technologies.
Backend is a bit more different, there are different "paradigms" of programming, and they kinda change how you write code. Some languanges follow one paradigm others mix and match a few. But to start you learn linear programming, it's the basic of every programming language.
The first backend language you learn is the hardest, because you have to learn the logic behind the concepts, besides the syntax of that language. But once you go to another backend language, you already know the basic concepts, you just gotta learn the new syntax, so it's much easier.
I recommend Python because it's a very simple syntax programming language, you can focus more on the logic instead of the syntax (which is a problem with older progrmaming languages, such as C, C++ and Java)
Python and JavaScript are good for newcomers imo