A common problem with float-based layouts is that the floats’ container doesn’t want to stretch up to accomodate the floats.

Solution: overflow: hidden;

check out the original article at

http://www.quirksmode.org/css/clearing.html

This is very useful and provided by YUI (Yahoo! UI Library).

I didn’t have any good impression toward to Yahoo! before.

But i am very glad they could share their UI database to open source.

Check out the original documents from here

http://developer.yahoo.com/yui

The following CSS selectors are considered workarounds and hacks for specific web browsers.

IE 6 and below
* html {}
IE 7 and below
*:first-child+html {} * html {}
IE 7 only
*:first-child+html {}
IE 7 and modern browsers only
html>body {}
Modern browsers only (not IE 7)
html>/**/body {}
Recent Opera versions 9 and below
html:first-child {}

Source: CSS Hacks

Usage
If you want to add 5px padding to a div element called #comments specifically for IE 7, then you can use the following hack:

*:first-child+html #comments {
padding: 5px;
}

But if you want to apply the padding just for IE 6, then the following will do the trick:

* html #comments {
padding: 5px;
}

Note that CSS Hacks are not recommended due to their dependence on browser bugs and therefore they should only be used as the last resort.

A more comprehensive article on CSS Hacks can be found here.

Source: http://eisabainyo.net/weblog/2007/04/18/css-hacks-for-ie-7-ie-6-opera-etc/

http://www.webcredible.co.uk/user-friendly-resources/css/css-tricks.shtml