Where can I find small jobs online for HTML, CSS experience?

Volunteer work, if someone you know, or a small business has NO web presence, offer your services with the agreement to use their site as a portfolio/promotional piece. IME, no one will turn that down, even when they have to pay for domain/hosting.

Craigslist! Freelancer beware! Ensure you draw up an agreement to recieve either half/full payment up-front, and be ready. When freelancing design work, be aware of client issues. Such as, not having any ideas of their own, and when you bring some to the table, they don't like what you've done, OR, they like what you've done, and a week later wants to revise the design a bit. In my professional experience, when you write a contract for any design work, take the time to assign a limit to the amount of revisions you will do, also an extra fee for any changes after approval.

NONPROFIT organizations! Check around for any 503c's that need design work done, this can be volunteer work as well or a flat fee. Plenty of 503c's to go around that ALWAYS need some form of help. The cost to hire a fully experienced designer/web guy/gal is probably WAYYYY outside of their funding scope, so a hundred or two that can increase their presence and visibility will go a LONG way.