I find CSS is the coding equivalent of herding cats.
Hmm. I wouldn't say it was quite that easy. At least you can bribe cats with nosh.
Back on topic...
HTML, the markup bit is straight-forward you'll learn all you need to know about that in under half an hour.
CSS? Well that has a rational basis and the basics aren't that horrid either. More effort than HTML but not beyond the wit of mortal man. However you'll find that that rational design has been undermined by rubbish / incomplete implementation of CSS standards in the various browsers as well as some browser manufacturers *cough* extending CSS so you will spend hours (if you're lucky) days (weeks) if you're not trying to create a CSS style that works the same and gives a good result in Safari, IE, Opera, FireFox, Pale Moon, {insert browser of choice here} and the rest of them.
What will really do your head in is the rest of the cruft that has been wrapped around, on-top, beside and underneath HTML & CSS. Oh yes that and the fact that no two browsers adhere to the various "standards" (there's a reason that word is in quotes - I'll let you guess why) in quite the same way, some don't even implement some "standards" at all. Had I mentioned that already? Never mind, it bears repeating. Many, many, many times.
And if you want something beyond simple presentation (read-only pages) then, well, we have to have the JavaScript talk and possibly the "Frameworks" talk as well. I'm sorry but JavaScript is, there is no polite way to put this, an horror story and the various "Frameworks" designed to, excuse me while I split my sides laughing, make things easier are no better. They are generally designed (designed? hah!) by spotty 14 year old oiks, have the supported lifetime of a Mayfly and each one comes in at least 4 mutually incompatible versions so that a JavaScript "library" written to work in/with one framework will almost certainly not work with another framework and almost as certainly won't work with either the previous version or the next but one version of the framework it is supposed to work with. And don't think mutually incompatible versions is the preserve of spotty oiks. Oh dearie me no. Step forward Microsoft and the change from (IIRC) MVC2 to MVC3. It might have been 3 to 4, but you take my point I trust?
Oh yes and there's JQuery for dynamically altering the appearance of a page and finding stuff on a page as well as Ajax and Fetch and $deity alone knows what other protocols / libraries for fetching data from servers.
Oh I nearly forgot. There's the added "joy" of millions (well hundreds of thousands at the very least) of badly written, unsupported and buggy JavaScript libraries riddled with potential security flaws out there ready to trap the unwary programmer and there's next to no _reliable_ vetting system to let you know what's usable and what's not. Yes NPM and Git I am looking at you.
We haven't even covered deploying the stuff you've created yet, but like any good horror flick we have to leave room for a sequel.
In short if value your sanity and can avoid having anything to with the uncontrolled maniac environment that is web UI development walk away now. If you really
must get involved well, come along in and join the rest of us frogs in the increasingly warm water.