Effects in webpages are more and more used, but often packaged in a big librairy such as Dojo, Scriptaculous, etc... Animator.js is a single little js allowing to make advanced effect very easly : 
ex15 = new Animator().addSubject(new CSSStyleSubject(
    $('ex15Button'),
    "width: 10em; background-color: rgb(256, 256, 256); font-style: normal",
    "width: 40em; background-color: #F39; font-style: italic"));
// note how you can use any unit, not just 'px'.
This simple code will make a fade between first styles attributes and second one.

Congratulation to the author for this very good library!