In this talk Monica Dinculescu takes a deep dive into webfonts and how the
font-display CSS property lets us control the way those fonts are rendered. She argues that there’s all sorts of huge performance gains to be had if we just spend a little bit of time thinking about the total number of fonts we load and how they’re loaded.
Also, Monica made a handy demo that gives an even more detailed series of examples of how the
font-display property works:
This depends a lot on how you are using your webfont, and whether rendering the text in a fallback font makes sense. For example, if you're rendering the main body text on a site, you should use font-display:optional. On browsers that implement it, like Chrome, the experience will be much nicer: your users will get fast content, and if the web font download takes too long, they won't get a page relayout halfway through reading your article.
If you're using a web font for icons, there is no acceptable fallback font you can render these icons in (unless you're using emoji or something), so your only option is to completely block until the font is ready, with font-display:block.