Unblock font loading in rustdoc.css

[workingjubilee]

rustdoc's font loading defaults to "auto", so browsers may block render.
But rustdoc's case prefers a faster TTI for scrolling, this means the
strictest font-display in use should be "swap". rustdoc's fonts do provide
notable legibility improvements but first-time users will have little trouble
reading without. This means "optional" is preferred.

The one exception is Source Serif Pro: it's a big difference for body text, so
"fallback" is preferred over "optional" to cause a (tiny) block.

This follows up on (but does not resolve) #20962, taking PageSpeed Insight's rec fairly directly. Supporting reading material: https://drafts.csswg.org/css-fonts-4/#font-display-desc

[GuillaumeGomez]

Thanks for the PR! Looks good to me, but before confirming, just a few questions: it first renders the page with the default system font I assume and once the font is loaded, it makes the switch, right? In my low level of understanding, it seems like it actually performs more operations and should be slower, but just to confirm: it's faster this way?

If "yes" is the answer to both questions, then it's good for me! :)

[ghost]

The time till you first see text is shorter.
So it feels like the page loads faster, which is what we want.

"optional" lets the user-agent decide if downloading the font is even necessary. Otherwise "swap" would be an alternative.

[workingjubilee]

Yes† and Yes.
† within sane limits

According to the draft spec and other docs I can find, it appears that most browsers implement a default behavior of beginning to render text anyways (invisibly) and then rerendering the text with the appropriate font once the font is loaded. So browsers just do a lot of ops anyways. These changes speed things up by allowing the browser to not render text twice if the fonts cannot be obtained for that initial rendering pass.

For fallback, if the fonts arrive within greater than 100ms but less than 3s the text is rerendered. This is the recommended behavior for getting body text to load fast (good) and try to guarantee it renders correctly i.e. with the desired font (good) but without mid-reading rerenders (bad). For other pieces of text, both fallback and optional seemed appropriate, so I chose optional, which allows (as mentioned) the choice of just not loading the fonts if they don't immediately arrive.

I would not recommend swap for the main body text. For other text, it risks less of a jarring effect.

On the second visit to a rustdoc page, however, the fonts should already be in cache, so they should load and render instantly. Unless the user is somehow accessing rustdoc webpages on an electronic potato that somehow has a browser installed, in which case the same logic applies.

[GuillaumeGomez]

Fine by me then, thanks!

@bors: r+ rollup

[bors]

📌 Commit 510fce1 has been approved by GuillaumeGomez