Flash Of Unstyled Content (FOUC)

A flash of unstyled content (FOUC) is an instance where a web page appears briefly with the browser’s default styles prior to loading an external CSS stylesheet, due to the web browser engine rendering the page before all information is retrieved. The page corrects itself as soon as the style rules are loaded and applied; however, the shift is quite visible and distracting.

https://en.wikipedia.org/wiki/Flash_of_unstyled_content