PageSpeed: Combine external CSS (deprecated)
Combining external stylesheets into as few files as possible cuts down on RTTs and delays in downloading other resources.
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Details from Google
It often makes sense to use many different CSS files during the development cycle, and then bundle those CSS files together as part of your deployment process. See the sidebar for recommended ways of partitioning your files. You would also need to update all of your pages to refer to the bundled files as part of the deployment process.