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

Internet fonts are sometimes horrible for internet efficiency and not one of the font loading methods are notably efficient to handle 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 internet efficiency, and there really are no good choices here. If you wish to use internet fonts your decisions 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 employ the fallback system font initially after which improve it to the net font when it downloads. Neither possibility has actually “received out” as a result of neither is actually 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 net 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 resolve the issue.

Plenty of 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’ll be able to, 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 net font downloads and characters increase (or contract) to fill within the subsequent area. Smashing Journal, like most publishers, makes use of internet fonts and the beneath screenshot exhibits the distinction between the preliminary render (with the fallback fonts), and the ultimate render (with the net 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 internet fonts. (Large preview)

Now, when put facet by facet, the net 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 format 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 remains to be rendered in invisible textual content), it a minimum of 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 attainable 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 want to attend a tiny bit longer to get the preliminary render right.

Utilizing font-display: optionally available Can Clear up FOIT And FOUT — At A Value

The opposite possibility is to make use of font-display: optionally available. This selection principally makes internet fonts optionally available, or to place in another 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 net 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 net font is that unimportant to the positioning, then it may be a good suggestion to simply take away them fully — which is even higher for internet efficiency!

Nonetheless, first impressions rely and to have that preliminary load with out internet fonts altogether appears a bit of bit an excessive amount of. I additionally suppose — with completely no proof to again this up by the way in which! — that it’ll give folks the impression, maybe subconsciously, that one thing is “off” concerning the web site and will impression how folks use the web site.

So, all font choices have their drawbacks, amongst with the choice to not use internet 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 internet font loading has been to make the fallback font nearer to the precise internet font to scale back the noticeable shift as a lot as attainable, in order that utilizing swap is much less impactful. Whereas we by no means will be capable of keep away from the shifts altogether, we cab do higher than within the screenshot above. The Font Style Matcher app by Monica Dinculescu is commonly cited in font loading articles and offers a unbelievable glimpse of what must be attainable right here. It helpfully means that you can 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 (high) and adjusted settings to offer a greater match (backside). (Large preview)

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

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

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

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

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

A Higher Answer Is Coming

So, that’s a short catch-up on the place issues stand as of right this moment. Nonetheless, there’s some smoke beginning to appear on the horizon.

As I discussed earlier, the principle situation with making use of the fallback styling variations was in including, after which eradicating them. What if we may 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 few proposed update to the font-face descriptors specification which incorporates 4 new descriptors: ascent-override, descent-override, line-gap-override and advance-override. You’ll be able to 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 mix of those 4 descriptors allowed us to override the format of the fallback font to match the net 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 adjustments with the upcoming size-adjust descriptor.

How does it work? Let’s say you’ve 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 mean you can get this nearer to the Lato font. It’s an additional @font-face declaration to put in writing, however actually quite a bit simpler than the hoops we needed to leap by way of earlier than!

General, there are 4 foremost @font-face descriptors included on this spec: size-adjust, ascent-override, descent-override, and line-gap-override with a couple 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 form of factor! Ideally, we may additionally expose these settings for frequent fonts to internet builders, e.g. perhaps give these hints in font collections like Google Fonts? That would definitely assist enhance adoption.

Now completely different working methods could have barely completely different font settings and getting these precisely proper is principally an inconceivable process, however that’s not the goal. The goal is to shut the hole so utilizing font-display: swap is now not such a jarring expertise, however we don’t must go to the extremes of optionally available or no internet 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 steady 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 would land very quickly.

For the time being, the spec has all types of disclaimers and warnings that it’s not prepared for real-time but, however it actually 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 using it, so the implementation doesn’t get caught as a result of too many individuals find yourself utilizing an earlier draft.

Chrome has said 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 appears 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 could possibly go a bit nearer to addressing your font loading woes and the CLS impression they trigger.

Smashing Editorial
(vf, il)

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