
CSS is a way to define how your site looks, while HTML's main goal is to define how your site is structured.
Therefor we can safely assume that CSS is all about colors, positions, heights, widths and all other kind of things that make your site appealing to the eye.
It is also a great technique to separate concerns for your development team. While a PHP developer does care about the structure of your HTML document (i.e. where to place the login form), he usually isn't concerned if a button is red or blue or maybe even has an overlay image.
So separating these things makes your team more productive and gives you better revenue in the end.
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