[tomjen3]

CSS is literally designed so that you can override previous definitions by loading your own stylesheet. Thats not just the standard, it is in the name (Cascading Style Sheet). The fact that no main-stream browser supports adding user specific styles out of the box is an issue with the browsers not acting as the user agents they are supposed to be and not really CSS.

[uniqueid]

CSS is reusable in some ways, but not in the sense that you can take a stylesheet from one site, apply it to another, and have a guarantee that the result will make sense.

This is partly because HTML has its own competing presentation features, and partly because there is an infinite number of cases for which a designer would have to write rules (eg: many html elements, that can also be nested)