CSSTidy

It is available as executeable file (available for Windows, Linux and OSX) which can be controlled per command line and as PHP script (both with almost the same functionality).

In opposite to most other CSS parsers, no regular expressions are used and thus CSSTidy has full CSS2 support and a higher reliability.

Features:

• Colors like "black" or rgb(0,0,0) are converted to #000000 or rather #000 if possible. Some hex-codes are replaced by their colour names if they are shorter.
• a{property:x;property:y;} becomes a{property:y;} (all duplicate properties are merged)
• margin:1px 1px 1px 1px; becomes margin:1px;
• margin:0px; becomes margin:0;
• a{margin-top:10px; margin-bottom:10px; margin-left:10px; margin-right:10px;} becomes a{margin:10px;}
• margin:010.0px; becomes margin:10px;
• all unnecessary whitespace is removed, depending on the compression-level
• All background-properties are merged
• All comments are removed
• The last semicolon in every block can be removed
• Missing semicolons are added, incorrect newlines in strings are fixed, missing units are added, bad colors (and color names) are fixed
• property:value ! important; becomes property:value !important;