CSS for js13k in 2022

There are only a handful of js13kGames entries each year that use CSS, but there have been tons of usful (yes, for games!) additions to the language over the last year or so. Hopefully me pointing some of them out can get more people interested!


Starting off simple, accent-color sets the color for UI controls like checkboxes and range sliders. Safari added it in version 15.4 (March 2022), so there's now full browser support! There's a great article on it by web.dev if you want to know more.

See the demo on CodePen

New Color Syntax

Hex notation (#ffffff or #fff) is usually best for game jams because of it's conciseness, however the other syntaxes have had some improvements recently, and are more likely to be worth using, especially for programatically-generated colors.

rgb() and hsl() with alpha

Both rgb() and hsl() now support an alpha channel without the a, and with spaces instead of commas:

/* old */
color: rgba(255, 204, 17, 0.5);
color: hsla(47deg, 100%, 53%, 0.5);
/* new */
color: rgb(255 204 17 / 0.5);
color: hsl(47deg 100% 53% / 0.5);

If you're hand-minifying, the newer syntax saves a few bytes, half because of the removal of a, and half because percentage values don't require the space after them.

/* old */
/* new */
color:rgb(255 204 17/.5); // 1B shorter!
color:hsl(47 100%53%/.5); // 2B shorter!

Hue Whiteness Blackness

hwb() is a newly implemented (full support since April 2022) color syntax, where H is hue, W is whiteness, B is blackness. It's nice to pick colors with, and minifies well. Worth mentioning that it does not support the old comma-based syntax.

color: hwb(47deg 7% 0%);
color:hwb(47 7%0%); // Minified

Flexbox & Grid Improvements

start Instead of flex-start

Safari is the only browser that requires the flex- part of align-items: flex-start for flexbox containers. All other browsers are fine with just align-items: start. If you're really in need of those few bytes, and still want to support Safari, you might be able to get away with a display: grid container instead.

gap Property for Flexbox

It took Safari until last year to implement this one, but it's great. Gone are the days of negative margins on the parent and positive margins on the children, or weird margins between siblings. Just use a grid or flexbox layout plus gap!

/* Ye old method */
.parent {
  margin: -1rem;
.child {
  margin: 1rem;

/* Another old one */
.child ~ .child {
  margin-left: 2rem;

/* New */
.parent {
  display: grid;
  gap: 2rem;


Two words: Frosted. Glass.

The backdrop-filter property applies a filter to an area behind an element. The most obvious example is backdrop-filter: blur() for iOS-style dialogs boxes, overlays, buttons, etc.

See the demo on CodePen

You can use any of the CSS <filter-functions> as values for backdrop-filter. Another helpful filter is invert(), as it can make UI elements like crosshairs stand out on any background.

See the demo on CodePen

A rudimentary tilt-shift effect can be created by combining a mask with backdrop-filter: blur().

First, the mask-image. We're going to use the mask shorthand, and it needs the -webkit- prefix as browsers are being slow at removing it. But even Firefox supports -webkit-mask.

This mask starts at the top as black (any color should work, checking if red can save a couple of bytes is on my todo list…) to apply the full filter effect, then fades to transparent so that towards the center the view the filter isn't applied

-webkit-mask: linear-gradient(black 10%, transparent 40% 60%, black 90%);

The .filter element must have some sort of background for the mask to be applied - so here we give it background: #0ff;

See the demo on CodePen

We can then swap out the background for a backdrop-filter - this one blurs slightly, and darkens and desatures with < 1 values.

backdrop-filter: blur(5px) brightness(0.8) grayscale(0.2);

See the demo on CodePen

This CSS-based city uses the effect to miniture-ify it:

See the demo on CodePen

backdrop-filter recently landed in Firefox 103, so it's now in all moderns browsers. The -webkit- prefix is still needed for Safari, but I'd recomend leaving that off for js13kGames, as losing the effect likely isn't a game-breaking bug.


Okay this one isn't CSS, it's HTML, but it can replace CSS. The HTMLElement inert property marks an element as non-interactable. It's better than pointer-events: none because not only is it shorter, but it also disables keyboard access, without having to tabIndex="-1" any interative children.

/* old */
<div class="ui" style="pointer-events: none">
  <div class="popup-modal></div>

/* new */
<div class="ui" inert>
  <div class="popup-modal></div>

Firefox will be getting inert next month in version 105, and all other moderns browsers already support it.


The inset CSS property is the new (since 2020) top, right, button, left shorthand.

.old {
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
.new {
  inset: 0;

Final Thoughts

Maybe let me know if you've learnt something or have been inspired to use CSS in your game! Via twitter or the js13kGames Slack

If you're wondering what an entire game made with CSS (instead of for example a <canvas> element) looks like, you can check out my js13kGames entry from 2021, 3DC5S. I also used some of the techniques mentioned here on my recent js1024 entries, mirror-puzzle and stacking-game.