Demos

Complex Spiral Eric Meyer shows what can be achieved in browsers, such as Mozilla, which support all of CSS1 (a standard published in 1997.)
Demo

MathML Mozilla supports the Mathematics Markup Language out of the box; Roger B. Sidje shows what can be achieved (not Mac OS X).
Demo

Eagle Shadow Mozilla supports PNG Alpha Masks and changing of image opacity through CSS, which allows cool effects like dynamically-calculated shadows.
Demo

Sticky Notes This implementation of "Sticky Notes" in a web page uses XBL to bind behaviours to page elements, so sadly it only works in Mozilla. But it's still cool.
Demo

MineHunter An entry for the "game in under 5k" awards, this is a full implementation of Minesweeper using only web standards.
Demo | Explanation

DHTML Menu Bar A complete menu bar with sub-menus, which can be dropped straight into a web page, using only CSS, DOM and JavaScript.
Demo | Explanation

Credits A lovely scrolling, fading text effect by Marcio Galli, using only JavaScript and DOM Level 1.
Demo