'Facebook currently has 6498 color declarations! You don’t get there unless there wasn’t a plan for maintainable markup from the beginning. Since that sounds like a big ol’ mess...'
Source: Our living style guide (Writing maintainable HTML/CSS - RJMetrics
CSS is more than just hacking stuff with Firebug and throwing !importants around.
A style guide is a great way to build some sense of the importance of adherence to a style.
Why:
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performance
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maintainability
Make sure you
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minify your site
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use preprocessors
How many people first come to CSS by way of having to fix the 6-10 core style sheets that define the site and want to fix link colour of something without bringing down a whole world of shit. Come on web artisans stop making CSS like it is 1999.