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

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...

🆕 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 No more outdated guesses from training data.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/

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:

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 👇

🦊 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/...

🦊 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: 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 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: 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: <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 👇 developer.mozilla.org/en-US/docs/...

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/...

🆕 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/...

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 👇 developer.mozilla.org/en-US/docs/...

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/...

✨ :only-child now matches elements with no parent! The :only-child selector has been updated — it can now match elements that have no parent at all, not just sole children. A subtle but useful spec alignment across all major browsers. Learn more 👇 developer.mozilla.org/en-US/docs/...

✨ :only-child now matches elements with no parent! The :only-child selector has been updated — it can now match elements that have no parent at all, not just sole children. A subtle but useful spec alignment across all major browsers. Learn more 👇 developer.mozilla.org/en-US/docs/...

Build smooth scroll experiences with CSS scroll snap 📜 Snap scrollable containers to specific positions — carousels, galleries, and paginated views with zero JavaScript. Learn more 👇 developer.mozilla.org/en-US/docs/...

📬 Check own properties safely with Object.hasOwn() A safer alternative to hasOwnProperty, works even on objects with a null prototype. Baseline since 2022 ✅ Learn more 👇 developer.mozilla.org/en-US/docs/...

Create frosted glass effects in CSS 🪟 backdrop-filter applies visual effects to the content behind an element - blur, brightness, contrast, and more. backdrop-filter: blur(10px) brightness(0.9); No JavaScript. No canvas tricks. Learn more 👇 developer.mozilla.org/en-US/docs/...

The CSS next-sibling combinator (+) selects an element only when it immediately follows another, same parent, no elements between them. img + p { font-weight: bold; } Learn more 👇 developer.mozilla.org/en-US/docs/...

🆕 Document.caretPositionFromPoint() is Newly Available! Give it an (x, y) coordinate and get back the exact DOM node and character offset. Perfect for click-to-edit interfaces. Check it out 👇 developer.mozilla.org/en-US/docs/...

🆕 The Popover API is Baseline, no JavaScript needed Tooltips, dropdowns, and menus with just HTML attributes. ⋅ popover attribute on any element ⋅ popovertarget to wire the trigger ⋅ Accessible by default, no ARIA hacks Learn more 👇 developer.mozilla.org/en-US/docs/...

Better error chaining is finally in JavaScript 🔗 `Error.cause` lets you attach the original error when rethrowing, keeping full context without losing the stack. Learn more 👇 developer.mozilla.org/en-US/docs/...

Speed up rendering with content-visibility: auto ⚡ This CSS property skips rendering off-screen content until needed, giving massive performance wins on long pages. ⋅ Pairs with contain-intrinsic-size ⋅ Zero-effort lazy rendering Learn more 👇 developer.mozilla.org/en-US/docs/...

React to element size changes, not just the window 📐 `ResizeObserver` fires when any element's dimensions change, perfect for responsive components. ⋅ Observe padding box, content box, or border box ⋅ Works with dynamic DOM changes Learn more 👇 developer.mozilla.org/en-US/docs/...

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/...

Run non-critical work without blocking the UI 💤 `requestIdleCallback()` schedules tasks during browser idle time. ⚠️ Limited Availability ⋅ Runs only when the main thread is free ⋅ Great for analytics, prefetching, and cleanup Learn more 👇 developer.mozilla.org/en-US/docs/...

JavaScript's date object has been tricky for years, but that is changing. The NEW Temporal API brings, 🌍 Easily handle time zones 📆 Precise date math 🕒 Parse ISO strings without errors ⌛ Durations, date ranges, and more. Start experimenting 👇 developer.mozilla.org/en-US/docs/...

JavaScript now has non-mutating array methods ✨ `toSorted()`, `toReversed()`, `toSpliced()`, and `with()` return new arrays instead of modifying the original. No more [...arr].sort() to avoid side effects. Learn more 👇 developer.mozilla.org/en-US/docs/...

🆕 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/...

Show focus rings only when needed 🎯 :focus-visible applies styles when an element receives focus via keyboard, but not on mouse clicks. Better UX for everyone. Read More 👇 developer.mozilla.org/en-US/docs/...

Transform streaming data on the fly 🔄 TransformStream sits between a readable and writable stream, letting you modify chunks as they pass through. Perfect for encoding, compression, or parsing. Learn more 👇 developer.mozilla.org/en-US/docs/...

Did you know about Clear-Site-Data header? 👀 One HTTP header to clear cookies, storage, or cache for your site. Perfect for logout flows. Learn more 👇 developer.mozilla.org/en-US/docs/...

Schedule work at the right time with queueMicrotask() ⚡ Run a callback after the current task but before rendering. More predictable than setTimeout(fn, 0), lighter than a Promise. Learn more 👇 developer.mozilla.org/en-US/docs/...

Format lists like a native speaker 🌐 Intl.ListFormat turns arrays into grammatically correct strings in any locale. Handles "and", "or", and narrow styles automatically. Learn more 👇 developer.mozilla.org/en-US/docs/...

Fit images perfectly with object-fit 🖼️ Control how replaced content (images, videos) fills its container. cover, contain, fill, scale-down, or none. No more stretched or squished images. Learn more 👇 developer.mozilla.org/en-US/docs/...

Parse and build query strings the easy way 🔗 URLSearchParams gives you a clean API for working with URL parameters. Get, set, append, delete, and iterate, no regex needed. Learn more 👇 developer.mozilla.org/en-US/docs/...

Happening tomorrow at Mozilla Berlin! 🚨 Firefox, open source & the future of the web. 🎤 Firefox Mobile open source contributions 🗺️ Firefox Roadmap & upcoming features 🤝 Networking + raffle 🗓️ Apr 28 // 18:00 RSVP now! 👇 www.meetup.com/berlin-mozi...

Support dark mode with one CSS property 🌙 color-scheme tells the browser your page supports light, dark, or both. Form controls, scrollbars, and system colors adapt automatically. > :root { color-scheme: light dark; } Learn more 👇 developer.mozilla.org/en-US/docs/...

Server-render Shadow DOM with declarative syntax ✨ Use <template shadowrootmode="open"> to create shadow roots in HTML. No JavaScript required for initial render. Perfect for SSR and web components. Learn more 👇 developer.mozilla.org/en-US/docs/...

Mozilla Meetup is back in Berlin! 🥳 🗓️ Apr 28 // 18:00 Firefox, open source & the future of the web. 🎤 Firefox Mobile open source contributions 🗺️ Firefox Roadmap & upcoming features 🤝 Networking + raffle RSVP now! 👇 www.meetup.com/berlin-mozi...

Transform iterators without converting to arrays 🔄 Iterator helpers like .map(), .filter(), .take(), and .drop() work lazily on any iterable. Process data efficiently without intermediate arrays. Learn more 👇 developer.mozilla.org/en-US/docs/...

Help us build the WebAssembly language reference on MDN! 🤝 We're documenting Wasm 1.0 instructions, reference-types, and SIMD, and looking for technical writers and reviewers to join. Join the webassembly channel on our Discord or start here 👇 github.com/mdn/mdn/iss...

🦊 Firefox 150 is here! What's new? 🎨 color-mix() accepts multiple colors 🎥 New :playing, :paused, :muted media pseudo-classes 📜 animation-range-start/end for scroll animations ♿ ariaNotify() for screen reader support and more... Release notes 👇 developer.mozilla.org/en-US/docs/...

Set operations are native in JavaScript 🔢 union(), intersection(), difference(), symmetricDifference() — no more manual loops for set math. > const a = new Set([1, 2, 3]); > a.intersection(new Set([2, 3, 4])); // Set {2, 3} Learn more 👇 developer.mozilla.org/en-US/docs/...

Simplify complex selectors with :is() and :where() ✨ Both group selectors to reduce repetition. The difference? :is() keeps specificity, :where() has zero specificity. > :is(h1, h2, h3) { color: navy; } > :where(h1, h2, h3) { margin: 0; } Learn more 👇 developer.mozilla.org/en-US/docs/...

Want to help make Wasm more accessible? Come join us! ✨

Responsive values without media queries 📏 clamp() sets a minimum, preferred, and maximum value in one function. Perfect for fluid typography and responsive sizing. > font-size: clamp(1rem, 2.5vw, 2rem); Learn more 👇 developer.mozilla.org/en-US/docs/...

Transform key-value pairs back into objects ↩️ Object.fromEntries() is the inverse of Object.entries(). Convert Maps, arrays, or URLSearchParams directly into plain objects. Learn more 👇 developer.mozilla.org/en-US/docs/...