A recent Mozilla Dev Derby provides eye-catching examples of what can be done with nothing more than HTML and CSS.
Mozilla's Dev Derby is a contest that runs every month. Each contest is themed. Prizes are modest -- an Android phone at the top end -- so nobody enters these competitions for the swag. They enter to learn and to stretch their muscles as developers.
Go ahead, play the game through. It doesn't take long (the longest part is the intro). I'll wait.
While some of the contestants are deeply experienced developers (such as Stu Nicholls, the creator of the image gallery linked above, who is proprietor of CSSPlay.co.uk), others are just starting out -- like the 16-year-old portfolio creator. Mozilla puts on these contests for their educational value. Even just reading through and understanding the invitation to a Derby -- see the links in the next paragraph -- provides a foundation for the contest's subject matter. From there, diving in to figure out what you want to create is bound to solidify the knowledge.
The Mozilla Developer Network is now accepting entries for the contests related to Geolocation (September), CSS Media Queries (October), and the Full Screen API (November). The Dev Derby is the place to start.