See the SitePoint logo? It's not an image! It's entirely done with a technique known as CSS Border Slants!
This complex technique lets you create shapes with diagonal edges (and even curved ones if you have a lot of time and patience!) by taking advantage of the diagonal edges that appear between the horizontal and vertical borders of a CSS block. These edges are clearly visible in the following diagram (which is also pure HTML and CSS).
The question is, why wouldn't you just use an image? Well, for particularly large shapes (where the equivalent image file would be large), CSS Slants can download faster. CSS Slants can also be dynamically modified with JavaScript using common Dynamic HTML techniques.
For more details on CSS Slants and some interesting examples, see the following resources: