Pure CSS speech bubbles – Nicolas Gallagher
http://nicolasgallagher.com/demo/pure-css-speech-bubbles/bubbles.html#
The demo page for Progressive enhancement: pure CSS speech bubbles.
For a detailed explanation view the CSS file. It is heavily commented.
All examples use simple, semantic HTML. No empty elements, no unnecessary extra elements, no JavaScript, no images (apart from that Twitter logo). Have a look at the source code.