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MDN Web Docs
@developer.mozilla.org
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The official MDN Web Docs account, now on Bluesky. We deliver the best web docs around.

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developer.mozilla.org
MDN Web Docs @developer.mozilla.org · Jun 15
🆕 Introducing the MDN MCP server!

Bring MDN's docs and browser compat data straight into your AI agent or IDE for accurate, up-to-date answers about the web platform.

No more outdated guesses from training data.

Learn more 👇
developer.mozilla.org/en-US/blog/introducing-mdn-mcp-server/
A headline Introducing MDN MCP server above a terminal window titled Light-dark CSS function for images, showing a prompt asking how to use the light-dark() CSS function for images and which browsers support it, with a note that the mdn tool was called 4 times, followed by an answer explaining that light-dark() takes two image values and works anywhere an image is accepted. The MDN logo appears on the right.
Reposted by MDN Web Docs
webdevs.firefox.com
When and how CSS custom property values are calculated might not be what you expect, and it has a big impact on your styles…
September 10, 2026 at 1:23 PM
Reposted by MDN Web Docs
webdevs.firefox.com
Major versions of Firefox now land every two weeks, so Firefox 155 dropped today with web platform features including:

🎉 Fully typed CSS attr()
🎉 CSS progress()
🎉 Promise.allKeyed() and allSettledKeyed()
🎉 Lots of new WebTransport stuff

…And more!

developer.mozilla.org/en-US/docs/M...
Firefox 155 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 155 that affect developers. Firefox 155 was released on September 1, 2026.
developer.mozilla.org
September 1, 2026 at 1:15 PM
Reposted by MDN Web Docs
webdevs.firefox.com
CSS attr() now works on all properties in Firefox 155. Here's how it works, and how to get the most out of it:
September 3, 2026 at 1:16 PM
Reposted by MDN Web Docs
webdevs.firefox.com
CSS sibling-count() and sibling-index() are newly available across engines! Here's how they work:
August 25, 2026 at 1:03 PM
Reposted by MDN Web Docs
webdevs.firefox.com
Firefox 154 landed today, and brings with it some great web platform features:

🎉 CSS text-box
🎉 CSS sibling-index() and sibling-count()
🎉 JS Iterator methods like chunks, window, includes, & join
🎉 HTTP No-Vary-Search

…And more! developer.mozilla.org/en-US/docs/M...
Firefox 154 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 154 that affect developers. Firefox 154 was released on August 18, 2026.
developer.mozilla.org
August 18, 2026 at 1:15 PM
Reposted by MDN Web Docs
webdevs.firefox.com
CSS text-box is newly baseline, and it's solves a common text layout issue. Here's how it works:
August 19, 2026 at 1:19 PM
Reposted by MDN Web Docs
webdevs.firefox.com
JS promise integration for WebAssembly landed in Firefox 153, making it easier to work with asynchronous calls from Wasm. Here's how it works:
August 4, 2026 at 12:14 PM
Reposted by MDN Web Docs
webdevs.firefox.com
JS text imports landed in Firefox 153, making it the first browser to support this feature. Here's how it works:
July 30, 2026 at 12:39 PM
Reposted by MDN Web Docs
paris-web.fr
Atelier #ParisWeb : le 24 septembre, @hellosct1.bsky.social vous expliquera comment participer à la traduction de @firefox.com ou encore la documentation @developer.mozilla.org.

www.paris-web.fr/2026/atelier...

#traduction #documentation #logicielLibre #communauté
Découvrez le bénévolat en traduction — Paris Web
L’interface d’un logiciel, l’assistance et la documentation pour le développement en langue française sont importantes pour toute personne, avec ou sans compétences techniques. Chez Mozilla, la …
www.paris-web.fr
July 10, 2026 at 6:34 AM
Reposted by MDN Web Docs
webdevs.firefox.com
Notification actions landed in Firefox 152, allowing you to add quick action buttons to your notifications. Here's how they work:
July 9, 2026 at 2:36 PM
Reposted by MDN Web Docs
webdevs.firefox.com
CSS field-sizing is newly baseline, meaning you can make various form fields size to their contents, without needing JavaScript.
June 17, 2026 at 2:46 PM
Reposted by MDN Web Docs
webdevs.firefox.com
Firefox 152 landed today, and includes web platform features like:

🎉 CSS field-sizing
🎉 Notification actions
🎉 unadjustedMovement pointer locks
🎉 getAnimations() pseudoElement option

…and more!

developer.mozilla.org/en-US/docs/M...
Firefox 152 release notes for developers (Beta) - Mozilla | MDN
This article provides information about the changes in Firefox 152 that affect developers. Firefox 152 is the current Beta version of Firefox and ships on June 16, 2026.
developer.mozilla.org
June 16, 2026 at 2:15 PM
developer.mozilla.org
🆕 Introducing the MDN MCP server!

Bring MDN's docs and browser compat data straight into your AI agent or IDE for accurate, up-to-date answers about the web platform.

No more outdated guesses from training data.

Learn more 👇
developer.mozilla.org/en-US/blog/introducing-mdn-mcp-server/
A headline Introducing MDN MCP server above a terminal window titled Light-dark CSS function for images, showing a prompt asking how to use the light-dark() CSS function for images and which browsers support it, with a note that the mdn tool was called 4 times, followed by an answer explaining that light-dark() takes two image values and works anywhere an image is accepted. The MDN logo appears on the right.
June 15, 2026 at 11:00 AM
Reposted by MDN Web Docs
webdevs.firefox.com
The Web Serial API landed in Firefox 151, allowing you to connect directly to microcontrollers, dev boards, 3D printers, power meters, and other serial-connected hardware from the web. Here's how it works:
May 28, 2026 at 2:37 PM
developer.mozilla.org
Ever wish HTTP status codes were easier to remember? 🖼️

Check out this MDN cheatsheet that pairs each status code with an emoji to help you visualize what each one means, from 200 OK ✅ to 418 I’m a teapot 🫖

Check it out 👇
A colorful infographic titled “MDN HTTP Code Cheatsheet with Emojis.” It categorizes HTTP status codes using emoji-enhanced lists grouped by status code classes.
May 18, 2026 at 8:00 PM
developer.mozilla.org
🦊 Firefox 150: web extensions can now use the Web Authentication API.

Extensions can create and retrieve WebAuthn credentials for any domain covered by their host permissions.

Read Now 👇
developer.mozilla.org/en-US/docs/...
Firefox 150 release notes feature a minimalist design with soft colors, highlighting new capabilities for web extensions.
May 18, 2026 at 10:00 AM
developer.mozilla.org
🦊 New in Firefox 150: the revert-rule CSS keyword.

Make a property behave as if the current rule didn't exist, letting a value from another matching rule take effect instead.

Read Now 👇
developer.mozilla.org/en-US/docs/...
Firefox 150 release notes with a smooth background design featuring soft waves in pastel colors.
May 15, 2026 at 3:00 AM
developer.mozilla.org
🦊 Firefox 150: light-dark() now accepts image values — not just colors.

Use it with images and gradients to adapt visuals based on color scheme.

background: light-dark(url(light.png), url(dark.png));

Read Now 👇
developer.mozilla.org/en-US/docs/...
Firefox 150 release notes feature a pastel background with waves, emphasizing updates and new features.
May 14, 2026 at 4:00 AM
developer.mozilla.org
🦊 Firefox 150 ships ariaNotify() on Document and Element.

Queue text to be announced by a screen reader — a more ergonomic and reliable alternative to ARIA live regions.

Read Now 👇
developer.mozilla.org/en-US/docs/...
Firefox 150 release notes highlighted with a light background and wavy design, inviting users to read more.
May 13, 2026 at 6:00 AM
developer.mozilla.org
🦊 Firefox 150: color-mix() now accepts more than two colors at once.

Mix any number of colors in a single declaration — no more nesting multiple calls.

Read Now 👇
developer.mozilla.org/en-US/docs/...
Firefox 150 release notes feature a pastel background with wavy patterns, highlighting the new color-mix() function for CSS.
May 12, 2026 at 8:00 PM
developer.mozilla.org
🦊 Firefox 150: is now supported.

Lazy-loaded images use their calculated layout size to select from srcset — no more duplicating media queries in HTML.

Read Now 👇
developer.mozilla.org/en-US/docs/...
Firefox 150 release notes feature a soft pastel background with text highlighting updates and a call to action to read more.
May 12, 2026 at 3:00 PM
developer.mozilla.org
Have you heard of CSS Subgrid? 🤔

A grid item can now pass its parent's grid lines to children, so nested elements align perfectly with the outer layout.

No more hacky workarounds for aligned nested grids.

Learn more 👇
developer.mozilla.org/en-US/docs/...
Code snippet showcasing CSS Subgrid implementation with defined grid properties for a main grid, items, and sub-items.
May 11, 2026 at 8:00 PM
developer.mozilla.org
🆕 The URL Pattern API is Newly Available!

Use it to match and extract parts of URLs, no need to reinvent routing logic. Supports literals, wildcards, named groups, and even regex constraints.

Learn how it works 👇
developer.mozilla.org/en-US/docs/...
Dark-themed code snippet showing URLPattern examples on MDN: two JavaScript blocks matching "/books" and "/books/:id" with console.log outputs.
May 11, 2026 at 10:00 AM
developer.mozilla.org
Firefox 150 ships 7 media pseudo-classes: :buffering, :muted, :paused, :playing, :seeking, :stalled, :volume-locked.

Style
Firefox 150 release notes feature a light background with pastel waves. Includes a clear call to action to read more about updates.
May 11, 2026 at 10:00 AM
developer.mozilla.org
Wait for all promises, even the failing ones 🔄

`Promise.allSettled()` waits for every promise to finish, success or failure.

⋅ Returns { status: 'fulfilled', value } or { status: 'rejected', reason }
⋅ Perfect for batch operations

Learn more 👇
developer.mozilla.org/en-US/docs/...
JavaScript code demonstrating `Promise.allSettled()` handling fulfilled and rejected promises, along with results logging output.
May 10, 2026 at 4:00 PM