JWt

This request allows CSS files to be served in the bootstrap phase, before the main page is served.
This allows the browser to load the CSS rules from the already served pages, and start processing them.
If the main page is then loaded, the CSS will then likely already be present, and its rules can be immediately applied.

If the CSS is loaded during the main page, the chances of flickering increase, as the rules are not all ready, and the browser is still fetching the CSS file or processing the rules.