> Actually, I generally would usually avoid CSS entirely, since the user's styles are generally more suitable.
I very much doubt that's true for the average user, who will simply be using the browser's built-in default – which to an not inconsiderable extent is rather constrained by backwards compatibility concerns rather than being truly optimised for usage as "the only stylesheet you'll ever need".
Besides, even with a boring plain old homepage-and-definitively-not-a-web-app it's not too difficult to run into the limitations of what you can achieve without some custom styling even purely in terms of basic layouting and typography.
(The "font-display" setting is also a setting that the user might want to change even if font downloading is enabled. My idea of "meta-CSS" would give you this and many other things for free. Meta-CSS would not inherently allow the timing to be adjusted, but it is a feature that can be added too if it is wanted.)
Another thing that I might want is to enable the document to specify its own fonts if it is SVG or PDF, but to disallow HTML documents from specifying their own fonts.