The 2021 Web Almanac: CSS

Eric A. Meyer; Shuvam Manna

Part I Chapter 1

CSS

Date published: 2021/12/01

Last updated: 2024/11/11

Written by Eric A. Meyer and Shuvam Manna

Reviewed by Chris Lilley, Jens Oliver Meiert, Estelle Weyl, Brian Kardell, Adam Argyle, and Lea Verou

Analyzed by Rick Viscomi

Edited by Shaina Hantsis

Introduction

CSS (Cascading Style Sheets) is one of the three main pillars for building pages on the web—with HTML, used to define the structure; and JavaScript, used to specify behavior and interactions, completing the triumvirate.

Compared to last edition, the 2021 Web Almanac offers a deeper insight into how the use of CSS differs in the realm of what we all think we need versus what we actually see in production. As the calls for more robust CSS features and the challenge of centering a <div> with CSS kept making the rounds on our blog posts, conference talks, and Twitter chatter, pages around the web offered us vastly contradicting results, betraying the fact that CSS has, perhaps, become old enough to put more thought on staying stable instead of going wild with the zaniest of toys.

While CSS-in-JS adoption grew to 3% of all pages crawled (a 1 percentage point jump from last year), cutting-edge Houdini features are still mostly confined to tutorials and example galleries. Responsiveness continued to be one of most engrossing priorities, with max-width and min-width being the top media queries, and calc() being the top CSS function most commonly in use to determine widths.

As users continue to throng to the web, let’s jump into the data that would give us a better insight into how we have been faring in painting the internet—a place that is a second home, a workspace, a garage, or a rabbit hole for the rest of us.

Usage

Figure 1.1. Distribution of stylesheet transfer sizes per page.

It isn’t the heaviest component of most pages, but CSS—like the rest of the web—continues to grow in size from year to year. The median web page loads around 70 KB of CSS, and at the upper end, the average size is just over a quarter of a megabyte. Compared to 2020, the median total CSS weight rose about 7.9%, and the 90th percentile just under 7%, while preserving the pattern seen last year that mobile CSS is a little smaller than desktop CSS across all percentiles.

Not every page was so constrained: the page with the greatest CSS weight loaded 64,628 KB. The biggest mobile CSS weight seems positively svelte in comparison: only 17,823 KB.

As in 2020, it was found that page weight wasn’t significantly driven by preprocessors. 17% of desktop pages and 16.5% of mobile pages included sourcemaps, up slightly from 15% last year. The consistent share of CSS including sourcemaps seems to indicate that the sourcemap share is due more to build tool usage than sourcemap adoption, as we would expect to see much bigger year-over-year changes to sourcemap usage otherwise.

As for what kinds of sourcemaps were used, the numbers were largely consistent with last year:

Sourcemap type	2020	2021
CSS files	45%	45%
Sass	34%	37%
Less	21%	17%

Figure 1.2. Sourcemap types in 2021 versus 2020.

While this could be taken as evidence that Sass continues to gain ground over Less, the changes are small enough that it’s difficult to call them significant, statistically or otherwise. Time, as always, will tell.

Figure 1.3. Distribution of the number of stylesheets per page.

In terms of the average number of stylesheets per page, whether embedded or external, the numbers this year are up only slightly from last year. The 50th through 90th percentiles went up by one each, while the 10th and 25th percentiles didn’t budge.

2,368

Figure 1.4. The largest number of external stylesheets loaded by a page.

Incredibly, this year’s record for the largest number of external stylesheets beat last year’s by nearly a factor of two: 2,368 versus 1,379 in 2020. Whoever’s done this, we beg you—combine some files and give your server a rest!

Figure 1.5. Distribution of the total number of style rules per page.

Number of stylesheets is one thing, but what about the number of actual style rules? Compared to last year, the lower percentiles rose a bit, while the highest barely budged. What is different in 2021 versus 2020 is that across nearly all percentiles, desktop pages have more rules on average than do mobile pages.

Selectors and the cascade

Understanding cascade is an incredibly important part of working with CSS. Even more so for instances when you’d see that the styles you had written for an element are not working at all.

CSS offers a number of ways of applying styles to pages, from classes, ids and using the all-important cascade to avoid duplicating styles.

Class names

Much like last year, the most popular class name on the web is active, and the fa, fa-* (the Font Awesome prefix), and wp-* (the WordPress prefix) class names make very strong showings. selected and disabled switched places in the lineup compared to last year, but the most heartening change was a 5% drop for clearfix, a sign that float-based layout continues to wane.

We were also heartened to see the placement of sr-only-focusable, which is a Bootstrap accessibility feature. It causes an element to be placed off-screen, yet remains accessible to screen readers.

IDs

Pages continue to use IDs, and at about the same rate as seen in 2020. Even the list of popular ID names is consistent: content sits in the top spot at about 14% of pages, followed by footer and header. These latter two IDs dropped about a percent versus last year, which isn’t really enough to say anything definitive about them other than, developers should replace them with the corresponding HTML elements <header> and <footer> whenever possible.

The IDs starting with rc- are part of Google’s reCAPTCHA system, most versions of which are inaccessible in various ways.

Attribute selectors

The most popular attribute selector continues to be type, which is most likely to be used in selecting form controls like checkboxes, radio buttons, text inputs, and so on.

Pseudo-classes and -elements

The ranking and distribution of both pseudo-classes and pseudo-elements was not greatly changed from the 2020 Web Almanac. A few rankings changed, but overall, things seemed highly static. Whether this represents a solidification of common practice, a snapshot of designer interests, or simply the nature of the analysis, is open to debate.

Figure 1.9. The most popular pseudo-classes.

Just as in 2020, the user-action pseudo-classes :hover, :focus, and :active took the top three spots, with all of them appearing in a minimum of two-thirds of all pages. Structural pseudo-classes put in a number of appearances, but one of the most interesting changes was :not(), the negation pseudo-class, becoming more popular than :visited and achieving a 50% share of pages.

One thing we did check specifically this year was the use of :focus-visible, a way to style elements in focus in a way that better matches user expectations. This capability landed in Chromium in 2020, Firefox in January 2021, and (as of publication) is available in Safari 15 behind an experimental flag. Likely reflecting its recent implementation status, it appeared in less than 1% of the pages analyzed. It will be interesting to see if that number changes over the next few years.

Figure 1.10. The most popular unprefixed pseudo-elements.

Most of the pseudo-elements in use are browser-specific ways of selecting things like specific interface components, parts of browser chrome, or highlighted text. Once we filtered those out, we found that ::first-letter is used on a very small number of pages, but still many more than ::first-line, which didn’t make it onto the chart at all. ::marker, a way of selecting list item markers like bullets or counters in an ordered list, has much less than 1% page share, yet still made it onto the list. We should note here that cross-browser support for ::marker is relatively new (October 2020). It will be interesting to see if use increases over the next few years.

`!important`

That old battleaxe !important maintains a toehold on the web, with its share of marked rules hardly changing at all compared to the 2020 Web Almanac.

If that seems like a lot, hold on to your IDEs: we found a mobile page with 17,990 rules marked !important! That just edged out the most-important desktop page, which had 17,648 specificity-busting rules. We sincerely, truly hope these were the result of a script or preprocessor gone wrong.

As for what !important gets applied to, as with last year, it’s display, with the rest of the chart falling in the same order as in 2020—with the exception of the last item on the chart, where position bumped off float.

The most popular properties targeted by !important. — Figure 1.12. The most popular properties targeted by `!important`.

Selector specificity

Percentile	Desktop	Mobile
10	0,1,0	0,1,0
25	0,2,0	0,1,3 (up 0,0,1)
50	0,2,0	0,2,0
75	0,2,0	0,2,0
90	0,3,0	0,3,0

Figure 1.13. Distribution of the median selector specificity per page.

Many CSS methodologies recommend that authors restrict themselves to single classes in order to squash all selectors’ specificity into a single layer that is more easily managed. The BEM methodology, for example, was found on 34% of all pages. The 10th percentile of median selector specificity shows further evidence of this type of thinking, where both desktop and mobile specificity averages at (0,1,0). This is in line with last year’s findings, as are nearly all the medians—with the exception of mobile’s 25th percentile, which rose a little bit.

Values and units

CSS provides multiple ways to specify values and units, either in set lengths or calculations based on global keywords.

Lengths

Figure 1.14. The most popular length units.

Whatever you may think of pixel lengths, it’s still the most popular length unit by far, appearing in about 71% of all pages. The second-place length unit, percentage, trailed pixels by an overwhelming distance.

Property	`px`	`<number>`	`em`	`%`	`rem`	`pt`
`font-size`	(▼1%) 69%	2%	(▼1%) 16%	(▼1%) 5%	(▲1%) 5%	2%
`line-height`	(▼5%) 49%	(▲3%) 34%	(▲1%) 14%	(▼1%) 2%	(▲1%) 1%	0%
`border-radius`	65%	(▼1%) 20%	3%	10%	(▲2%) 2%	0%
`border`	71%	(▲1%) 28%	2%	0%	0%	0%
`text-indent`	(▼1%) 31%	(▲1%) 52%	8%	(▼1%) 8%	0%	0%
`gap`	(▼8%) 13%	(▲2%) 18%	(▼1%) 0%	0%	(▲7%) 69%	0%
`vertical-align`	(▼11%) 18%	12%	(▲11%) 66%	4%	0%	0%
`grid-gap`	(▲3%) 66%	(▼1%) 10%	9%	(▼1%) 0%	(▼2%) 14%	0%
`padding-inline-start`	(▼7%) 26%	(▲2%) 7%	(▲4%) 66%	0%	0%	0%
`mask-position`	0%	0%	(▼1%) 49%	(▲1%) 51%	0%	0%
`margin-inline-start`	(▼7%) 31%	(▲5%) 51%	(▲1%) 15%	(▲2%) 2%	1%	0%
`margin-block-end`	(▲1%) 5%	(▲7%) 38%	(▼9%) 56%	0%	(▲1%) 1%	0%

Figure 1.15. Distribution of length types per property.

Where things become interesting is in the breakdown of exactly how the various length units are used. To pick one example, the most common length unit used on line-height is pixels, followed by <number> values (which includes all instances of unitless zero length values). ems are the most popular length unit for vertical-align and padding-inline-start.

The positive and negative figures given in parentheses next to the figures in this table show change from 2020 results. In almost every property we analyzed, pixels became less popular as compared to the uses of other length units, with just two exceptions. The biggest change by far was in vertical-align, with an 11-point shift from pixels to ems as the unit of choice when the supplied value was a length, as opposed to a keyword like baseline.

Figure 1.16. The most popular font-relative length units.

Although em maintains a huge dominance over rem when it comes to sizing fonts, there are signs of change: there was a seven-point swing from em to rem between 2020 and 2021.

Figure 1.17. The units (or lack thereof) used on zero-length values.

There are a few properties that allow bare <number> units (e.g., line-height), but <length> values have a special case where a length of zero does not require a unit. When we looked at all zero-length values, almost 88% of them omitted the unit. Nearly all of those zero lengths that included a unit used pixels (0px). This was a nice result to see, since any length of zero doesn’t need a unit and including one is fairly pointless. We hope the share of unitless zero values will grow in the future.

Calculations

The most popular properties using calc() functions. — Figure 1.18. The most popular properties using `calc()` functions.

As in past years, the most popular usage of calc() is to set widths, although the share of calc() values in width dropped a full 20 points as compared to 2020. This seems most likely to reflect an expansion of calc() use in other properties, rather than a contraction of its use for width.

The most popular length units used in calc() functions. — Figure 1.19. The most popular length units used in `calc()` functions.

Although pixel units didn’t shift at all in terms of their usage in calculations, percentages lost a bit of ground compared to the long tail of other units, falling four points since 2020.

The most popular operators used in calc() functions. — Figure 1.20. The most popular operators used in `calc()` functions.

As with last year, when it comes to calculation operators, subtraction is the clear favorite, and barely shifted its share of usage. There were much bigger changes in the second and third spots, where addition vaulted ahead of division, gaining six points while division dropped a similar amount.

The number of unique units used in calc() functions. — Figure 1.21. The number of unique units used in `calc()` functions.

calc() values remain relatively simple, with the overwhelming preponderance of calculations using two different units, such as to subtract pixels from the calculated result of a percent value. A total of 99% of all calc() expressions use either one or two unit types.

Global keywords

Figure 1.22. Usage of global keyword values.

The use of global keywords such as initial rose significantly as compared to the 2020 Web Almanac. While inherit only gained a couple of points, initial rose about eight points, and unset around 10 points. Even revert managed to lift itself up a point.

Colors

Figure 1.23. The most popular color value formats.

Despite the availability of a wide number of color value types, the #RRGGBB syntax that has been with us since the days of Netscape 1.1 is still used in half of all color declarations. The CSS innovation of the #RGB shorthand came in second, at a quarter of color values. In other words, a solid 75% of all color values are expressed using hexadecimal RGB syntax. The third-place format, rgba(), points to the likely reason authors go beyond the classic hexadecimal format: to get access to alpha values. (Indeed, though both their shares are tiny, hsla() is more popular than hsl(), just as rgba() is much more common than plain rgb().)

In color formats where the value has historically used commas inside a functional syntax—for example, rgba(0, 0, 0, 1)—authors may now drop the commas and separate colors from alpha with a slash (thus, rgb(0 0 0 / 1). Since 2020, this comma-less syntax has doubled its usage share, going from 0.12% to 0.25% of all functional color syntax.

Keyword	Desktop	Mobile
`transparent`	82.24%	82.93%
`white`	7.97%	7.59%
`black`	2.44%	2.29%
`red`	2.23%	2.17%
`currentColor`	1.94%	2.03%
`gray`	0.68%	0.64%
`silver`	0.56%	0.55%
`grey`	0.39%	0.37%
`green`	0.32%	0.31%
`blue`	0.15%	0.12%
`whitesmoke`	0.12%	0.11%
`orange`	0.12%	0.10%
`lightgray`	0.08%	0.08%
`lightgrey`	0.07%	0.07%
`yellow`	0.07%	0.06%
`gold`	0.04%	0.03%
`magenta`	0.03%	0.03%
`Background`	0.02%	0.03%
`Highlight`	0.02%	0.03%
`pink`	0.03%	0.03%

Figure 1.24. The most popular named-color keyword values.

In the realm of just the named colors, transparent is still the faraway favorite, with around 82% of all named color keyword usage. The familiar and comfortable white, black, and red total another 12% or so, and currentColor comes in fifth with a half-percent rise over its 2020 numbers.

In last year’s Web Almanac, there was a note about “the once-deprecated—now partially un-deprecated—system colors like Canvas and ThreeDDarkShadow” being just barely in use. This is still true, but oddly, there are now two such values in the top 20 instead of just one (Highlight). That said, both occur in the realm of tiny, tiny numbers of pages, so such shifts are probably unremarkable.

79%

Figure 1.25. Percentage of display-p3 colors that lie outside the sRGB space.

The usage of the display-p3 color space remains about as vanishingly small as was found in 2020, probably because it’s only supported in Safari (both desktop and mobile) as of this writing. Desktop and mobile use roughly tripled, to 90 and 105 pages, respectively. In the cases where color(display-p3) was used, it was with good reason: 79% of the colors expressed using display-p3 on mobile were colors that cannot be represented in the sRGB color space. Until the color() function becomes more widely supported by browsers, the web will remain stuck in sRGB, which permits about two-thirds of the colors that screens can actually display.

Images

They say a picture is worth a thousand words, but byte wise, they often cost an order of magnitude or two more. While there are a myriad of approaches to embedding images with JavaScript, or include them with the HTML scaffolding, here we looked at how CSS-loaded images are used.

Formats of images in CSS

First, here’s a breakdown of the image formats we looked for, and how often each format appeared:

Figure 1.26. Distribution of the formats of external images loaded via CSS.

PNG was the clear favorite, with a surprisingly close clustering of GIF, SVG, and JPG following behind. The fairly new WEBP format accounted for only 3.7% of images loaded by CSS, and the tiny slice at the top corresponds to unrecognized values and the ICO format.

We did not attempt to determine whether any of the images were animated.

Please also note that this analysis only covers the images loaded by CSS: we did not check the HTML to see what was being loaded there. Thus, the following results cannot be taken as a metric of how heavy web pages are, or even how heavy CSS is or is not. It can only show how much CSS-loaded images contribute to a page’s total weight.

Number of images in CSS

Figure 1.27. Distribution of the number of external images loaded via CSS.

We found that most CSS doesn’t result in a lot of image loads: the lower two percentiles came in at one image each, and even the 90th percentile hovered around 10 images, across all image types.

6,089

Figure 1.28. The largest number of external images loaded by a page’s CSS.

We did find one site where the desktop CSS loaded 6,088 PNG images. The mobile version of the site actually added an image, bringing it to 6,089 PNGs. We hope they were all small and color-indexed for efficiency’s sake.

Weight of images in CSS

The number of images is one thing, but how much they weigh is at least as important—loading a single 10 MB background is worse than loading ten 100 KB pictures, after all, even with server compression factored in.

Figure 1.29. Distribution of the total weight in KB of external images loaded via CSS.

All told, things were not as bad as we’d feared going in: the median page’s CSS loads a total of 16 KB or so in images. It was also encouraging to see that overall, mobile image loading via CSS was consistently a bit lower than desktop—a sign that CSS developers do keep the limitations of mobile contexts at least somewhat in mind.

314,386

Figure 1.30. The heaviest total weight of images loaded via CSS, in KB.

Sometimes, anyway. We did find a page where the total weight of the images loaded by CSS was a gargantuan 314,386.1 KB—a third of a gigabyte.

Percentile	JPG	PNG	GIF	(other)	SVG	WebP
10	4.5	0.7	0.5	0.3	0.4	1.7
25	28.2	2.2	1.7	0.3	0.6	14.2
50	114.3	7.0	3.7	0.3	1.7	39.6
75	350.7	36.4	8.3	48.1	5.4	133.9
90	889.3	173.6	13.0	229.2	20.0	361.8

Figure 1.31. Distribution of the total weight in KB of external images loaded via CSS on mobile pages, by image format.

When we broke down the image weights by format, we discovered a fascinating tidbit: at the 90th percentile, GIF images were actually lighter on average than even SVG files.

It was also interesting, though perhaps not surprising, that the heaviest image format was JPG. This is likely because JPG is favored for those big splashy photographs one so often sees across the tops of home pages and so forth, and even with compression and other optimization tricks, all those pixels do add up.

Gradients

Property	Desktop	Mobile
`background`	62%	62%
`background-image`	62%	61%
`-webkit-mask-image`	5%	5%
`--*`	1%	1%
`mask-image`	1%	1%
`border-image`	1%	1%

Figure 1.32. Percentage of properties given gradient image values.

The share of pages using CSS gradients was roughly the same as last year: 77% of desktop pages and 76% of mobile pages. The properties on which they were used did change, however: while still the overwhelming favorites, background and background-image were the properties to which about 62% of gradients were assigned.

Figure 1.33. The most popular types of gradient image values.

Linear gradients continue to be the clear favorite, maintaining the 5-to-1 lead over radial gradients seen in the 2020 Web Almanac.

When prefixed versions of gradients (e.g., -webkit-linear-gradient) were included, the resulting graph looked basically the same as last year’s.

Some other things we found in analyzing gradient values:

The median number of color stops in gradients is just two, except at the 90th percentile, where the four stops was the median.
Hard color stops—that is, gradients where two color stops were placed at the same position—occurred in just over half of all gradients.
Color-stop interpolation (a.k.a. “midpoints”) were used in 21% of all gradient instances.

Figure 1.34. The linear gradient with the most color stops.

We also saw a dramatic reduction at the top end of gradient complexity. Last year, the gradient with the largest number of color stops had 646 stops. This year, the winner had only 81 color stops.

Layout

We have come a long, long way from using tables to create layouts on the web to a time when we have a number of options to choose from—Flexbox, Grid, and Multicolumn, as well as old chestnuts like floats, positioning and even CSS table properties. We did a simple search of stylesheets to see which property and value combinations were present, and came up with the following figures.

Note that this doesn’t chart primary layout methods—we are not claiming here that 93% of the pages we analyzed are laid out using absolute positioning! Rather, what the chart says is that position: absolute appeared in the styles for 93% of the page we analyzed, even if that was just to put an icon in a corner or place bits of content -9999px offscreen. Similarly, display: grid may have appeared in 36% of page’s styles, but that doesn’t mean 37% of all pages are Grid pages, just that the combination appeared somewhere in the stylesheet.

The rest of this section is where more in-depth analyses were done, looking not just for property-value combinations, but for evidence of actual usage on pages.

Flexbox and Grid adoption

Figure 1.36. Adoption of Flexbox and Grid layout on mobile devices.

The adoption of Flexbox and grid continues to grow. In 2019, Flexbox adoption was 41%; in 2020, it was 63%. This year, Flexbox hit 71% on mobile and 73% on desktop. Grid, in the meantime, has been doubling each year of the Web Almanac, from 2% to 4% and now 8%. Note that, in contrast to the previous section, what is measured here is the percentage of pages that are actually using Flexbox or Grid for layout, as opposed to the pages that simply have some sort of Flexbox or Grid property in their stylesheet.

Usage of different Grid layout techniques

Digging into the various Grid properties, we discovered a few interesting patterns.

About 15% of all Grid pages used grid-template-areas to define named areas of the grid.
When we looked for square brackets in Grid templates, which would indicate the presence of named Grid lines, we found a little fewer than 10,000 pages out of the seven million or so analyzed.

We also analyzed Flexbox layouts to see which ones set the flex grow and shrink values to zero, and then set all the flex item widths to be something static, like percentage or pixel widths. These are referred to as “Grid-like Flexbox,” and we found that just over a quarter of all Flexbox layouts met these criteria. Given the complexity of the analysis, it is entirely possible that we missed many cases. Still, it seems clear that designers are strongly interested in grid-style layouts, and this could drive migration to Grid in the coming years.

Multicolumn

20%

Figure 1.37. The percentage of pages using multicolumn layout.

Even though multicolumn layout is a bit fraught on the web, where it can force users to scroll down to the bottom of a column and then back up to the top of the next column, we detected multicolumn use on 20% of the pages we analyzed, which is a 5% rise over the 2020 Web Almanac. We continue to be surprised to see it on so many pages, and even more surprised to see its adoption increasing.

Box sizing

Distribution of the median number of border-box declarations per page. — Figure 1.38. Distribution of the median number of `border-box` declarations per page.

The principles of the original W3C box model continue to be rejected: when we looked to see how many pages were using box-sizing: border-box, it was an overwhelming 90%, up around 5% from 2020. Almost half of all pages analyzed apply border-box sizing to every element on the page via the universal selector (*). This “one sizing fitted to all” approach may help explain why the median number of border-box declarations per page is so low across the bottom three percentiles.

In addition, about a quarter of pages apply box-sizing to checkboxes and radio buttons.

Transitions and animations

Animations continue to be widely used, with the animation property appearing on 77% of all mobile and 73% of all desktop pages analyzed. It’s even more popular cousin, transition, is used on 85% of all mobile and 90% of all desktop pages.

Figure 1.39. The most popular properties given transition effects.

Among those transitions, the most common application is to all animatable properties using the all keyword (whether explicitly or by default), which occurred in 46% of the analyzed pages. Just behind that is opacity, at 42% of all pages containing transitions.

Figure 1.40. Distribution of transition durations.

We took a look at the duration and delay times of those transitions. Even at the 90th percentile, the median transition duration was just half a second.

Figure 1.41. Distribution of transition delays.

The highest median transition delay was 1.7 seconds, but even more interestingly, the 10th percentile median delay was about not quite one-third of a negative second, indicating that a large number of transitions are started partway through the resulting animation (which is what negative delays cause to happen).

A closer look at the range of transition durations and delays revealed some seriously lengthy spans of time. The largest duration value we found was 9,999,999,999,999,996 seconds, which corresponds to almost 317 million years. Put another way, if that duration were used in a horizontal scroll transition of If the Moon Were Only 1 Pixel, it would take just over two centuries to scroll to the right by a single pixel. This, however, pales in comparison to the longest transition delay we found: a value in milliseconds that equals not quite 31.7 quintillion years.

Figure 1.42. Adoption of transition timing functions.

As for the timing functions used during the transitions, the clear leader is the default value, ease. There’s a virtual tie for second between ease-in-out and linear, but the surprise was our fourth-place finisher, cubic-bezier. This seems most likely to come from a library or some sort of tool, because while it’s possible to learn how to construct cubic Bézier curves by hand, very few people bother to do so (nor is there much reason why they should).

Okay, but what kinds of animations are being performed? To determine this, we classified various animation labels by the type of animation being performed. For example, animations labeled fa-spin, spin, spinner-spin, and so on were classified as “rotate” animations, and these were the most popular.

Figure 1.43. The most popular types of animation.

One reason for the high ranking of “unknown/other” is the animation label a, which was around 6-7% of all named animations. (The most likely companion to these, b, had a 2% prevalence.)

The weak showing of “move” and “slide” style animations might seem surprising but remember: these are specifically types of animation. Transitions driven by the transition property are not represented in this sample. It is highly likely that many simple movements (and fades) are handled with transitions, and animation is reserved mostly for more complex effects.

Responsive design

Making a site that copes well with all the different screen sizes wherein you can now browse the web has become significantly easier with the advent of built-in tools like Flexbox and Grid, which are further enhanced by using media-queries.

Media features in use

When authors build their media queries, they most often test the width of the viewport. max-width and min-width were the most popular queries by far, the same as in 2020. There was no ranking change in the third and fourth place results either.

Figure 1.44. The most popular features used as media queries.

Where we did see a notable change was in the ranking of the prefers-reduced-motion query. This query placed 7th in 2020, with a share of 24%; this year, with a share of 32%, it’s up to fifth, where it just missed edging out orientation.

We also saw newcomers come and go at the bottom of the list. pointer, a query which checks to see if the display device’s primary input mechanism is a pointing device such as a mouse and which placed 19th last year, fell off the chart as it slipped to 21st place. The hover media feature, on the other hand, entered the chart at 20th place. hover is used to test if the display device’s primary input mechanism can cause a hover state in elements on the page.

Both queries have a similar aim, which is (put simply) to figure out if the device being used to display the page is mouse-driven or not. Combined with a mobile-first design philosophy, where desktop styles are added to override the default mobile styling, one can see how queries like pointer or hover would be useful. While it’s too soon to say if one or the other will become dominant, the trends this year swung toward hover.

This year also saw the debut of prefers-color-scheme, coming in at 7%. This may be due to iOS devices adding dark mode support since last year’s report, but in any event, it’s good to see that designers are starting to take color scheme preferences into account.

Common breakpoints

As in 2020, the most common breakpoints by far are at 767 and 768 pixels, which correspond suspiciously well with the resolution of an iPad in portrait mode. We found 767px was overwhelmingly used as a maximum-width breakpoint and only rarely as a minimum-width value. 786px, by contrast, was quite often used as both a minimum and maximum breakpoint.

Figure 1.45. The most popular media query breakpoints.

Beyond the 767-768 range, the next most popular breakpoints were at 600 and 1,200 pixels, and close behind that was 480 pixels.

Lest you think we converted all the breakpoint queries to pixels, we’re sorry to say we did not: these are the straight values from stylesheets. Out of all the breakpoints we analyzed, the first non-pixel value on the list is 48em, which came in at 76th on the ranking list, appearing in 1% of desktop and 2% of mobile styles. The next em-based value, 40em, is found in 85th place.

Properties inside media queries

So, what do authors actually style inside these media query blocks? The most often property to set is display, followed closely by color, width, and height.

Figure 1.46. The most popular properties to be changed via media queries.

One of the most notable changes between 2020 and 2021 was the fall of font-size as a property set inside media blocks. In 2020, it appeared in 73% of all media blocks, placing fifth on the list. This year, it appeared in around 60% of all media blocks, coming in 12th on the list.

margin-right and margin-top had even bigger falls, going from 8th and 9th to 25th and 17th, respectively. These sorts of shifts strongly imply a change in a common framework or piece of software—a change in the default WordPress theme would be one example, though we cannot say if this is the exact source of the change.

Feature queries

Feature queries (@supports) continue to grow in usage. In 2019, 30% of pages were found to use them, and last year it was 39%. In 2021, almost 48% of pages are using feature queries to decide which CSS to apply in what contexts.

So, what do authors condition CSS upon? Sticky positioning was far and away the most popular query, accounting for over half of all feature queries.

The most popular CSS features to be queried with @supports. — Figure 1.47. The most popular CSS features to be queried with `@supports`.

Only 3% of feature queries checked for Grid support, which translates to 261,406 pages querying Grid support. Given that we found grid layout in use on 2.7 million mobile pages and 2.3 million desktop pages, if our numbers are accurate, it appears that the vast majority of Grid layouts are deployed without fallbacks.

Custom properties

Figure 1.48. Change in custom property usage, 2019-2021.

Over the three years of the Web Almanac, custom properties (also known as CSS variables) have seen one of the greatest surges in usage. In 2019, usage was around 5% of all sites, and last year that had shot up to nearly 20% mobile and 15% desktop. This year, we found custom properties being defined on 28.6% of all mobile pages, and 28.3% of desktop pages. Even more, we found that 35.2% of mobile and 35.6% of desktop pages contained at least one var() function value.

Naming

Figure 1.49. The most popular custom property names.

The first thing we checked was, “What are developers calling their custom properties?” As it turned out, the prevalence of WordPress came out here, with the top entry being a link-coloring custom property defined by the WP core.

After that, a lot of color names were found. It might seem odd that anyone would need to define a custom value for --blue when the named color blue is sitting right there, but in practice, developers are assigning custom shades to their basic color names. So rather than --blue: blue, we see declarations like --blue: #3030EA.

Usage

Figure 1.50. The most popular properties to be given a custom-property value.

In addition to all the custom properties named after colors, the four most popular properties to be the recipients of custom-property values (using the var() function) are all setting color in one way or another.

Figure 1.51. Distribution of types of custom property values.

Each custom property gets a CSS value of one type or another. For example, --red: #EF2143 is assigning a color value to --red, whereas --multiplier: 2.5 is assigning a number value. We found that the most popular value type was colors, followed by dimensions (lengths), and then fonts families, whether singly or in groups.

Complexity

It’s possible to include custom properties in the values of other custom properties. Consider this example from the 2020 Web Almanac:

:root {
  --base-hue: 335; /* depth = 0 */
  --base-color: hsl(var(--base-hue) 90% 50%); /* depth = 1 */
  --background: linear-gradient(var(--base-color), black); /* depth = 2 */
}

As the comments in the previous example show, the more of these sub-references are chained together, the greater the depth of the custom property.

Figure 1.52. Distribution of median custom property depth.

Perhaps unsurprisingly, the clear majority of custom properties had a value depth of zero: they did not include the values of other custom properties in their own values. Nearly a third have one level of depth, and beyond that, there are almost no custom-property values with a depth of two or more.

As in 2020, we also checked the selectors in which custom-property values were used. Almost 60% were set on the root element (using either the :root or html selectors), and around 5% were applied to the <body> element. The rest were applied to some descendant of the root element other than <body>. This means around two-thirds of all custom properties are used as what are, in effect, global constants. This is in line with the results seen last year.

Internationalization

English is written horizontally, and the characters are read from left to right. But for languages such as Arabic, Hebrew and Urdu, among others, are written right to left and then there are languages and scripts—such as Mongolian, Chinese, and Japanese—which can be written in vertical lines, from top to bottom. Owing to this, things can get quite complicated. Both HTML and CSS provide ways to handle this.

Direction

Text direction can be explicitly enforced using the CSS property direction. We found it in use on the <html> element in 11% of all pages, and on the <body> element on 3% of pages. (Note that there may be overlap there, as we did not check for duplicate results.)

Of those pages that used CSS to set direction, 92% of <html> elements and 82% of <body> elements were set to ltr (left-to-right). Overall, we found rtl (right-to-left) used on only 9% of pages that set a direction in CSS. This is more or less to be expected, given that most languages are not right-to-left.

Logical and physical properties

Another CSS feature useful for internationalization are the “logical” properties like margin-block-start, padding-inline-end, and so on, as well as values such as start and end for properties like text-align. These properties and values allow box features to be tied to the direction of text flow, rather than physical directions like top, right, bottom, and so on.

Figure 1.53. Distribution of property types of logical properties.

As of mid-2021, only 4% of pages were found to be using logical properties of any kind. Of the pages that did, about 33% were using it to set text-align to start or end. Another 46% or so (combined) were setting logical margins and padding. Again, note that there could be overlap in these figures.

Ruby

In addition to directionality and logical features, CSS also offers internationalization support via CSS Ruby, a collection of properties used to affect the layout of interlinear annotation, which are short runs of text alongside the base text. Its usage is vanishingly small: only 8,157 desktop pages and 9,119 mobile pages were found to be using it—less than 0.1% of all pages analyzed.

CSS and JS

Figure 1.54. Distribution of CSS-in-JS libraries.

While the topic of “CSS in JS” is good for at least a Twitter flame war or two, its use in the wild continues to be very small. This year, we found that about 3% of pages are using some form of CSS-in-JS, up from 2% in 2020. Furthermore, nearly all of it comes from libraries built for the purpose, and more than half of that usage is from the Styled Components library.

Houdini

In some ways, CSS Houdini represents the opposite of the CSS-in-JS approach: it allows authors to mix a little JS into their CSS. Perhaps in part due to slow implementation (in browsers that aren’t based on Blink) of core parts of the specification, Houdini has struggled to find its feet. We find that it’s effectively not used on the open web in 2021: only 1,030 desktop pages and 1,175 mobile pages show evidence of animated custom properties, a feature of Houdini. This is a threefold increase over the 2020 findings, but it looks like it will still be some time before Houdini finds an audience.

Sass

One of the great advantages of CSS preprocessors is that they can reveal what’s missing in CSS itself, and can thus be a guide to how CSS should be extended in the future. This has already happened before, with variables being so popular in preprocessors that CSS eventually added custom properties to its repertoire. Other features of preprocessors, like color modifications and nested selectors, are also finding their way into the base language. This is why we devote a section of this chapter to seeing how developers are using Sass, one of the most popular preprocessors on the web today.

The Sass functions we found in use largely mirrored those found in the 2020 Web Almanac, albeit with some changes in the specific percentages. When classified by type, we found that 28% of all Sass functions were those that modify colors (e.g., darken, mix) and a further 6% were used to read color components (e.g., alpha, blue).

Figure 1.66. The most commonly used Sass flow control structures.

The desire for conditional behavior can be seen in the fact that the if() function placed third on the list, at 15% of all Sass functions.

This same desire can be seen even more clearly in the use of Sass’s flow control structures, like @if. Literally two-thirds of all Sass stylesheets use @if, and more than half use @for or @each (or both). This popular capability was recently added to CSS. By contrast, only 2% use the @while structure.

Figure 1.67. The prevalence of rule-nesting in Sass.

Another of Sass’s major draws is the ability to nest rules inside other rules and thus avoid having to write repetitive selector patterns. This capability is under development for native CSS, and our analysis shows why: 87% of all Sass stylesheets use a detectable form of rule nesting. Implicit nesting, which does not require special characters, was not measured.

Conclusion

In the end, the 2021 Web Almanac tells the story of a technology that’s stable but still evolving. We saw very few instances of major shifts between last year’s Almanac and this year’s—some practices and web features are clearly growing, while others are beginning to fade, but overall, there was a very strong sense of continuity.

Does this mean CSS has become stagnant? Hardly: new layout methods are gaining ground, and major new capabilities are being developed, many of them based on practices worked out in the realm of preprocessors. We would not think to claim that CSS is “solved” or that the best possible practices have already been worked out. As practitioners gain ever more experience, changes will come to both CSS the language and CSS the practice. These changes may be gradual rather than sudden, steady rather than disruptive, but this is what we expect in any mature technology.

We look forward to seeing how CSS will grow over the years to come.

Property	Desktop	Mobile
`font-variant`	3,098,211	3,641,216
`font-variant-numeric`	153,932	166,744
`font-variant-ligatures`	107,211	112,345
`font-variant-caps`	81,734	86,673
`font-variant-east-asian`	20,662	20,340
`font-variant-position`	5,198	5,842
`font-variant-alternates`	4,876	5,511