Hi,
Its all do with specifity and weight.
To overide the id you will need to be more specific.
Code:
div#myblock a.myclass { color:#00f; }
This style is more specific div#myblock a { color:#0f0; } and targets a elements with more weight than a simple class like a.myclass { color:#00f;}.
Therefore you have to use make your targetting morw specific. Id's carry more weight than classes and there is an algotithm used to determine which style is applied and the more specific you make the targetting then the higher the weighting of the style.
See here for more info:
http://www.meyerweb.com/eric/css/link-specificity.html
Paul
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