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MDN Web Docs @developer.mozilla.org · 25/09/2026

🆕 Customize MDN's browser compat tables! Press ⚙️ Settings on any compat table to choose which browsers and engines you see, and hide the ones you don't need. Your picks are saved in your browser. Try it 👇 developer.mozilla.org/en-US/docs/W...

MDN's browser compatibility table for the HTML popover attribute. Its columns cover desktop and mobile browsers, showing which versions support the attribute and its "hint" value. A "Settings" button sits above the table.

The Browser compatibility settings dialog on MDN. Browsers appear as chips grouped under Desktop, Mobile, Server and XR, and each one can be added to or removed from the table.

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Firefox for Web Developers @webdevs.firefox.com · 10/09/2026

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

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Firefox for Web Developers @webdevs.firefox.com · 01/09/2026

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...

developer.mozilla.org

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.

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Firefox for Web Developers @webdevs.firefox.com · 03/09/2026

CSS attr() now works on all properties in Firefox 155. Here's how it works, and how to get the most out of it:

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Firefox for Web Developers @webdevs.firefox.com · 25/08/2026

CSS sibling-count() and sibling-index() are newly available across engines! Here's how they work:

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Firefox for Web Developers @webdevs.firefox.com · 18/08/2026

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...

developer.mozilla.org

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.

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Firefox for Web Developers @webdevs.firefox.com · 19/08/2026

CSS text-box is newly baseline, and it's solves a common text layout issue. Here's how it works:

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Firefox for Web Developers @webdevs.firefox.com · 04/08/2026

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

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Firefox for Web Developers @webdevs.firefox.com · 30/07/2026

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

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Firefox for Web Developers @webdevs.firefox.com · 09/07/2026

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

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Firefox for Web Developers @webdevs.firefox.com · 17/06/2026

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

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Firefox for Web Developers @webdevs.firefox.com · 16/06/2026

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...

developer.mozilla.org

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.

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MDN Web Docs @developer.mozilla.org · 15/06/2026

🆕 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/

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Firefox for Web Developers @webdevs.firefox.com · 28/05/2026

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:

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MDN Web Docs @developer.mozilla.org · 18/05/2026

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.

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MDN Web Docs @developer.mozilla.org · 18/05/2026

🦊 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.

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MDN Web Docs @developer.mozilla.org · 15/05/2026

🦊 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.

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MDN Web Docs @developer.mozilla.org · 14/05/2026

🦊 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.

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MDN Web Docs @developer.mozilla.org · 13/05/2026

🦊 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.

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MDN Web Docs @developer.mozilla.org · 12/05/2026

🦊 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.

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MDN Web Docs @developer.mozilla.org · 12/05/2026

🦊 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/...

Firefox 150 release notes feature a soft pastel background with text highlighting updates and a call to action to read more.

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MDN Web Docs @developer.mozilla.org · 11/05/2026

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.

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MDN Web Docs @developer.mozilla.org · 11/05/2026

🆕 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.

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MDN Web Docs @developer.mozilla.org · 11/05/2026

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/...

Firefox 150 release notes feature a light background with pastel waves. Includes a clear call to action to read more about updates.

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MDN Web Docs @developer.mozilla.org · 10/05/2026

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.

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MDN Web Docs @developer.mozilla.org · 10/05/2026

✨ :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/...

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MDN Web Docs @developer.mozilla.org · 09/05/2026

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MDN Web Docs @developer.mozilla.org · 08/05/2026

📬 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/...

Code snippet demonstrating the use of Object.hasOwn() in JavaScript to check property existence in various scenarios.

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MDN Web Docs @developer.mozilla.org · 08/05/2026

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/...

Code snippet showcasing various CSS `backdrop-filter` functions like blur, brightness, contrast, and saturation against a blue background.

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MDN Web Docs @developer.mozilla.org · 08/05/2026

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/...

Code snippet illustrating CSS sibling selection with `li:has(+ li:last-of-type)` and style rules for color and font weight.

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MDN Web Docs @developer.mozilla.org · 07/05/2026

🆕 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 graphic displays the syntax for the `caretPositionFromPoint()` function in a code snippet format against a blue background.

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MDN Web Docs @developer.mozilla.org · 07/05/2026

🆕 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/...

Code snippet demonstrating a simple popover with a toggle button, showcasing HTML attributes for functionality.

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MDN Web Docs @developer.mozilla.org · 06/05/2026

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/...

Code snippet demonstrating error handling in JavaScript using `try`, `catch`, and `Error.cause` for maintaining context during rethrowing.

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MDN Web Docs @developer.mozilla.org · 06/05/2026

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/...

CSS snippet detailing the `content-visibility` property with keyword and global values for efficient rendering practices.

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MDN Web Docs @developer.mozilla.org · 05/05/2026

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/...

Code snippet demonstrates using `ResizeObserver` to adjust element width dynamically based on its content size.

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MDN Web Docs @developer.mozilla.org · 05/05/2026

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MDN Web Docs @developer.mozilla.org · 05/05/2026

Just a couple, here and there. ¯\_(ツ)_/¯

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MDN Web Docs @developer.mozilla.org · 05/05/2026

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/...

Syntax for `requestIdleCallback()` shown on a dark background, highlighting its parameters for managing browser idle time efficiently.

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MDN Web Docs @developer.mozilla.org · 04/05/2026

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/...

Code snippet demonstrating temporal date precision in JavaScript, highlighting differences in epoch milliseconds output.

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MDN Web Docs @developer.mozilla.org · 01/05/2026

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/...

Code snippet demonstrating the `toSorted()` method, showcasing sorting of arrays without mutating the original arrays.

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MDN Web Docs @developer.mozilla.org · 01/05/2026

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MDN Web Docs @developer.mozilla.org · 30/04/2026

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/...

CSS code snippet demonstrating styles for input and select elements with the :focus-visible pseudo-class for improved accessibility.

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MDN Web Docs @developer.mozilla.org · 30/04/2026

It was just there when we found it. 😅

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MDN Web Docs @developer.mozilla.org · 30/04/2026

This isn’t a technical constraint of the locale system, it’s an editorial convention tied to regional style guides. Most US style guides recommend the Oxford comma (the comma before “and” in a list) British style guides are less prescriptive and often omit the Oxford comma unless it’s for clarity.

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MDN Web Docs @developer.mozilla.org · 30/04/2026

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/...

Code snippet demonstrating chaining of multiple ReadableStreams with TransformStream in a JavaScript context.

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