MDN Web Docs

The official MDN Web Docs account. We deliver the best web docs around.

[MDN Web Docs @MozDevNet]

🆕 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No more outdated guesses from training data.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 👇
https://t.co/yMeR7tVwkM

[Firefox for Web Developers @FirefoxWebDevs]

When and how CSS custom property values are calculated might not be what you expect, and it has a big impact on your styles…

[Firefox for Web Developers @FirefoxWebDevs]

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!

https://t.co/zGtRtYX1uk

[Firefox for Web Developers @FirefoxWebDevs]

CSS text-box is newly baseline, and it's solves a common text layout issue. Here's how it works: https://t.co/1bb6hCSjAZ

[Firefox for Web Developers @FirefoxWebDevs]

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! https://t.co/4w76Li3X1B

[Firefox for Web Developers @FirefoxWebDevs]

JS promise integration for WebAssembly landed in Firefox 153, making it easier to work with asynchronous calls from Wasm. Here's how it works:

[Firefox for Web Developers @FirefoxWebDevs]

JS text imports landed in Firefox 153, making it the first browser to support this feature. Here's how it works:

[Firefox for Web Developers @FirefoxWebDevs]

Notification actions landed in Firefox 152, allowing you to add quick action buttons to your notifications. Here's how they work:

[Firefox for Web Developers @FirefoxWebDevs]

CSS field-sizing is newly baseline, meaning you can make various form fields size to their contents, without needing JavaScript.

[Firefox for Web Developers @FirefoxWebDevs]

Firefox 152 landed today, and includes web platform features like:

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

…and more!

https://t.co/zVWp2byhYX

[Firefox for Web Developers @FirefoxWebDevs]

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:

[MDN Web Docs @MozDevNet]

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 👇

[MDN Web Docs @MozDevNet]

🦊 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 👇
https://t.co/kTQrRg7pkN

[MDN Web Docs @MozDevNet]

🦊 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 👇
https://t.co/kTQrRg7pkN

[MDN Web Docs @MozDevNet]

🦊 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 👇
https://t.co/kTQrRg7pkN

[MDN Web Docs @MozDevNet]

🦊 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 👇
https://t.co/kTQrRg7pkN

[MDN Web Docs @MozDevNet]

🦊 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 👇
https://t.co/kTQrRg7pkN

[MDN Web Docs @MozDevNet]

🦊 Firefox 150: <img sizes="auto"> is now supported.

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

Read Now 👇
https://t.co/kTQrRg7pkN

[MDN Web Docs @MozDevNet]

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 👇
https://t.co/Hl0hD1oIN7

[MDN Web Docs @MozDevNet]

Firefox 150 ships 7 media pseudo-classes: :buffering, :muted, :paused, :playing, :seeking, :stalled, :volume-locked.

Style <audio> and <video> elements based on their playback state.

Read Now 👇
https://t.co/kTQrRg7pkN