Reading: CSS Link in body.
You need to take care when using layout systems where content dictates layout (such as tables & flexbox) to avoid content-shifting during load. This isn’t a new problem, but progressive rendering can expose it more frequently. You can hack flexbox to behave, but CSS grid is a much better system for overall page layout (flexbox is still great for smaller components).
I teach web design. I write books on HTML5 games development. I host classes on mobile web and app development. I made animation with Adobe Flash (now Animate).