Is there a job market for people with just CSS3/HTML5 knowledge but nothing more?
EDIT: disclaimer - it's not me, it's a friend, but still the question is as important. it warms the heart to see how much support and good spirit HN community has, he'll love to read the replies. thanks.
But think a bit about the bigger picture here - do you want to make a career out of this? And if so, where do you want to go with it? HTML/CSS is a good place to start because it represents the fundamental visual building blocks of web apps. It's also in a bit of the middle ground - all the way at the front of the "web stack" code-wise, and also very closely related to design.
no_more_death suggests that you bone up on design and gradually learn to code. If you want to eventually work on your own, as a freelancer or co-founder, this is good advice. If you're looking to make a career at larger companies, or want to focus more on short-term job prospects, I'd suggest you pick one of the two - buckle down on learning either design or code.
What compels you about HTML/CSS? Is it the logic of boiling down visual concepts into their fundamental parts? If so, start playing with Javascript (with a jQuery cushion) to see how you can manipulate these parts in all sorts of ways, and go down the code rabbit hole. If you're more into the visual side of it, the creativity of being able to come up with any design you like and turn it into a website, start messing around in Photoshop and checking out some design resources online (see no_more_death's extensive list).
In any case, good luck, and let me know if you have any more questions. Yes, you can probably get by in life knowing only HTML & CSS and never starve. But I hope you branch out a bit and check out some of the bigger picture, because it's really quite cool :)