PHP function flush() allows you to send your partially ready HTML response to the browser so that it will start to fetch the components while your backend is still busy with the rest of the HTML page. You can experience this effect when you have busy backends or light front ends.

Though the speed increase will actually depend on the server-side processing, page weight, and size of your CSS files. It is a simple optimization, and nothing wrong in doing it.

To allow the browser to fetch CSS and JavaScript files in parallel, without disturbing the backend, it is recommened to use the function  in between  and  tags. If you are usingWordPress, then you have to edit header.php file probably.

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