Tuesday May 25, 2021 By David Quintanilla
CSS Font Descriptors — Smashing Magazine

Net fonts are sometimes horrible for net efficiency and not one of the font loading methods are significantly efficient to deal with that. Upcoming font choices could lastly ship on the promise of creating it simpler to align fallback fonts to the ultimate fonts.

Font loading has lengthy been a bugbear of net efficiency, and there really are no good choices here. If you wish to use net fonts your selections are principally Flash of Invisible Textual content (aka FOIT) the place the textual content is hidden till the font downloads or Flash of Unstyled Textual content (FOUT) the place you utilize the fallback system font initially after which improve it to the online font when it downloads. Neither choice has actually “received out” as a result of neither is de facto passable, to be trustworthy.

Wasn’t font-display Supposed To Clear up This?

The font-display property for @font-face gave that option to the online developer whereas beforehand the browser determined that (IE and Edge favored FOUT prior to now, whereas the opposite browsers favored FOIT). Nonetheless, past that it didn’t actually remedy the issue.

Quite a few websites moved to font-display: swap when this primary got here out, and Google Fonts even made it the default in 2019. The pondering right here was that it was higher for efficiency to show the textual content as rapidly as you possibly can, even when it’s within the fallback font, after which to swap the font in when it lastly downloads.

I used to be supportive of this back then too, however am more and more discovering myself pissed off by the “hydration impact” when the online font downloads and characters broaden (or contract) to fill within the subsequent house. Smashing Journal, like most publishers, makes use of net fonts and the under screenshot exhibits the distinction between the preliminary render (with the fallback fonts), and the ultimate render (with the online fonts):

Two screenshots of a Smashing Magazine article with different fonts. The text is noticeably different sized and an extra sentence can fit in when the web fonts are used.
Smashing Journal article with fallback font and with full net fonts. (Large preview)

Now, when put facet by facet, the online fonts are significantly nicer and do match with the Smashing Journal model. However we additionally see there’s fairly some distinction within the textual content structure with the 2 fonts. The fonts are very completely different sizes and, due to this, the display content material shifts round. On this age of Core Web Vitals and Cumulative Format Shifts being (fairly rightly!) acknowledged as detrimental to customers, font-display: swap is a poor alternative due to that.

I’ve reverted again to font-display: block on websites I take care of as I actually do discover the textual content shift fairly jarring and annoying. Whereas it’s true that block received’t cease shifts (the font continues to be rendered in invisible textual content), it no less than makes them much less noticeable to the consumer. I’ve additionally optimized by font-loading by preloading fonts that I’ve made as small as potential by self-hosting subsetted fonts — so guests typically noticed the fallback fonts for under a small time frame. To me, the “block interval” of swap was too brief and I’d actually favor to attend a tiny bit longer to get the preliminary render right.

Utilizing font-display: non-obligatory Can Clear up FOIT And FOUT — At A Value

The opposite choice is to make use of font-display: non-obligatory. This selection principally makes net fonts non-obligatory, or to place in a different way, if the font isn’t there by the point the web page wants it, then it’s as much as the brwoser to by no means swap it. With this selection, we keep away from each FOIT and FOUT by principally solely utilizing fonts which have already been downloaded.

If the online font isn’t accessible then, we fall again to the fallback font, however the subsequent web page navigation (or a reload of this web page) will use the font — because it ought to have completed downloading now. Nonetheless, if the online font is that unimportant to the location, then it could be a good suggestion to simply take away them utterly — which is even higher for net efficiency!

Nonetheless, first impressions depend and to have that preliminary load with out net fonts altogether appears a bit of bit an excessive amount of. I additionally suppose — with completely no proof to again this up by the best way! — that it’s going to give individuals the impression, maybe subconsciously, that one thing is “off” in regards to the web site and will affect how individuals use the web site.

So, all font choices have their drawbacks, amongst with the choice to not use net fonts in any respect, or utilizing system fonts (which is limiting — however perhaps not as limiting as many think!).

Making Your Fallback Font Extra Carefully Match Your Font

The holy grail of net font loading has been to make the fallback font nearer to the precise net font to cut back the noticeable shift as a lot as potential, in order that utilizing swap is much less impactful. Whereas we by no means will be capable to keep away from the shifts altogether, we cab do higher than within the screenshot above. The Font Style Matcher app by Monica Dinculescu is usually cited in font loading articles and provides a improbable glimpse of what ought to be potential right here. It helpfully permits you to overlay the identical textual content in two completely different fonts to see who completely different they’re:

Font Style Matcher screenshots showing two sets over text overlaid on each other with the top having big differences and the bottom having the text very similar.
Font Style Matcher screenshots with the default, similar settings for 2 fonts (prime) and adjusted settings to present a greater match (backside). (Large preview)

Sadly, the difficulty with the font model matching is that we will’t have these CSS types apply solely to the fallback fonts, so we have to use JavaScript and the FontFace.load API to use (or revert) these model variations when the online font hundreds.

The quantity of code isn’t large, nevertheless it nonetheless appears like a bit of bit extra effort than it ought to be. Although there are different benefits and prospects to utilizing the JavaScript API for this as defined by Zach Leatherman in this fantastic talk from way back in 2019 — you possibly can scale back reflows and deal with data-server mode and prefers-reduced-motion although that (be aware nevertheless that each have since been uncovered to CSS since that speak).

It’s additionally trickier to deal with cached fonts we have already got, to not point out variations in varied fallback types. Right here on Smashing Journal, we attempt plenty of fallbacks to make one of the best use of the system fonts completely different customers and working techniques have put in:

font-family: Mija,-apple-system,Arial,BlinkMacSystemFont,roboto slab,droid serif,segoe ui,Ubuntu,Cantarell,Georgia,serif;

Realizing which font is used, or having separate changes for every and guaranteeing they’re utilized accurately can rapidly turn into fairly complicated.

A Higher Answer Is Coming

So, that’s a quick catch-up on the place issues stand as of in the present day. Nonetheless, there’s some smoke beginning to appear on the horizon.

As I discussed earlier, the principle problem with making use of the fallback styling variations was in including, after which eradicating them. What if we might inform the browser that these variations are just for the fallback fonts?

That’s precisely what a brand new set of font descriptors being proposed as a part of the CSS Fonts Module Level 5 do. These are utilized to the @font-face declarations the place the person font is outlined.

Simon Hearne has written a couple of proposed update to the font-face descriptors specification which incorporates 4 new descriptors: ascent-override, descent-override, line-gap-override and advance-override. You may play with the F-mods playground that Simon has created to load your customized and fallback fonts, then play with the overrides to get an ideal match.

As Simon writes, the mixture of those 4 descriptors allowed us to override the structure of the fallback font to match the online font, however they solely actually modify vertical spacing and positioning. So for character and letter-spacing, we’ll want to supply extra CSS. Nonetheless, it now modifications with the upcoming size-adjust descriptor.

How does it work? Let’s say you have got the next CSS:

@font-face {
  font-family: 'Lato';
  src: url('/static/fonts/Lato.woff2') format('woff2');
  font-weight: 400;

h1 {
    font-family: Lato, Lato-fallback, Arial;

Then what you’d do is to create a @font-face for the Arial fallback font and apply adjustor descriptors to it. You’ll get the next CSS snippet then:

@font-face {
  font-family: 'Lato';
  src: url('/static/fonts/Lato.woff2') format('woff2');
  font-weight: 400;

@font-face {
    font-family: "Lato-fallback";
    size-adjust: -9900.00%;
    ascent-override: 96%;
    src: native("Arial");

h1 {
    font-family: Lato, Lato-fallback, sans-serif;

This implies thata when the Lato-fallback is used initially (as Arial is a native font and can be utilized right away with none extra obtain) then the size-adjust and ascent-override settings will let you get this nearer to the Lato font. It’s an additional @font-face declaration to jot down, however definitely lots simpler than the hoops we needed to bounce by earlier than!

General, there are 4 important @font-face descriptors included on this spec: size-adjust, ascent-override, descent-override, and line-gap-override with a number of others nonetheless being thought-about for subscript, superscript, and different use instances.

Malte Ubl created a very useful tool to automatically calculate these settings given two fonts and a browser that helps these new settings (extra on this in a second!). As Malte factors out, computer systems are good at that type of factor! Ideally, we might additionally expose these settings for frequent fonts to net builders, e.g. possibly give these hints in font collections like Google Fonts? That will surely assist enhance adoption.

Now completely different working techniques could have barely completely different font settings and getting these precisely proper is principally an unimaginable activity, however that’s not the purpose. The purpose is to shut the hole so utilizing font-display: swap is now not such a jarring expertise, however we don’t have to go to the extremes of non-obligatory or no net fonts.

When Can We Begin Utilizing This?

Three of those settings have already been shipped in Chrome since model 87, although the important thing size-adjust descriptor shouldn’t be but accessible in any secure browser. Nonetheless, Chrome Canary has it, as does Firefox behind a flag so this isn’t some summary, far-off idea, however one thing that might land very quickly.

For the time being, the spec has all kinds of disclaimers and warnings that it’s not prepared for real-time but, nevertheless it definitely feels prefer it’s getting there. As all the time, there’s a steadiness between us, designers and builders, testing it and giving suggestions, and discouraging the usage of it, so the implementation doesn’t get caught as a result of too many individuals find yourself utilizing an earlier draft.

Chrome has acknowledged their intent to make size-adjust available in Chrome 92 due for release on July 20th presumably indicating it’s nearly there.

So, not fairly prepared but, however seems to be prefer it’s coming within the very close to future. Within the meantime, have a play with the demo in Chrome Canary and see if it may possibly go a bit nearer to addressing your font loading woes and the CLS affect they trigger.

Smashing Editorial
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