A Python 3 implementation for client-side web programming Without a doubt, you've seen a clock like this in demos of HTML5
However, right click and view the source of this page...
Speed of execution is similar to CPython for most operations.
The gallery highlights a few of the possibilities, from creating simple document elements to drag and drop and 3D navigation. A wiki lists some applications using Brython.
You can also take a look at presentations made in various conferences.