- You load the content of a web page.
- You load the styles of a web page.
In the time period after (1) but before (2), the content is being displayed without its proper styles. This time period is usually brief; it's a "flash".
In an attempt to avoid unstyled content, front-end developers may choose to hide all content until it is fully loaded, at which point a load event handler is triggered and the content appears.
Basically, to only show content once the styles have loaded and it could be displayed properly.
Is this a good idea? Are there usability benefits, or is it just aesthetic? The downside is that it takes longer before a user will see content on their page.