New to the web platform in September

Discover some of the interesting features that have landed in stable and beta web browsers during September 2026.

Rachel Andrew

Published: October 2, 2026

Stable browser releases

It's the first month of bi-weekly Chrome and Firefox releases. That means Chrome 153, Chrome 154, Firefox 155, Firefox 156, Firefox 157, and Safari 27 released to stable during September 2026. This post takes a look at the new features landing across browsers, including a large number of features reaching Baseline Newly available.

The progress() CSS function becomes Baseline

Firefox 155 adds support for the progress() CSS math function, making it Baseline Newly available.

The progress() function returns a <number> representing the position of one calculation between a start value and an end value. Typically, the result is clamped between 0 and 1, which makes it useful for driving fluid sizing, opacity, or colors based on container or viewport dimensions. Both Firefox 155 and Safari 27 also add support for the no-clamp keyword option, allowing the returned progress value to extend below 0 or above 1.

.card {
  opacity: progress(100vw, 320px, 1200px);
}

Browser Support

  • Chrome: 138.
  • Edge: 138.
  • Firefox: 155.
  • Safari: 26.

Source

Relative alpha colors with alpha() become Baseline

Firefox 155 and Safari 27 both add support for the alpha() relative color function from CSS Color 5.

The alpha() function takes an origin color and returns that color with an adjusted alpha (transparency) channel while preserving its color channels and color space:

.overlay {
  background-color: alpha(var(--brand-color) / 50%);
}

With Chrome 151, Firefox 155, and Safari 27 all supporting alpha(), this function is now Baseline Newly available.

Browser Support

  • Chrome: 151.
  • Edge: 151.
  • Firefox: 155.
  • Safari: 27.

Source

The revert-rule CSS keyword becomes Baseline

Safari 27 adds support for the revert-rule CSS-wide keyword.

While revert rolls back the cascade to the user or user-agent origin and revert-layer rolls back to a previous cascade layer, revert-rule rolls back a property's value to what it would have been if the current style rule weren't present. With support in Chrome, Firefox, and now Safari 27, revert-rule is Baseline Newly available.

Browser Support

  • Chrome: 148.
  • Edge: 148.
  • Firefox: 150.
  • Safari: 27.

Source

<image> values in light-dark() become Baseline

Safari 27 adds support for using <image> values inside the light-dark() CSS function.

Previously limited to <color> values, light-dark() can now switch between two images—such as url() images or CSS gradients—based on the active light or dark color-scheme. With support across Chrome, Firefox, and Safari 27, <image> values in light-dark() are now Baseline Newly available.

Browser Support

  • Chrome: 150.
  • Edge: 150.
  • Firefox: 150.
  • Safari: 27.

Scroll anchoring with overflow-anchor becomes Baseline

Safari 27 enables scroll anchoring and adds support for the overflow-anchor CSS property.

Scroll anchoring adjusts the scroll position automatically when DOM changes above the visible viewport would otherwise cause content to jump while the user is reading or interacting with the page. You can use overflow-anchor: none to opt specific elements out of acting as anchor nodes. With Safari 27 joining Chrome and Firefox, scroll anchoring is now Baseline Newly available.

Browser Support

  • Chrome: 56.
  • Edge: 79.
  • Firefox: 66.
  • Safari: 27.

Source

The font-width property and @font-face descriptor become Baseline

CSS Fonts 4 replaced font-stretch with font-width as the canonical name for selecting normal, condensed, or expanded font faces, keeping font-stretch as a legacy alias.

Firefox 155 adds support for the font-width CSS property, the font-width @font-face descriptor, and CSSStyleDeclaration.fontWidth. Chrome 154 also adds the font-width descriptor in @font-face rules and the width property on FontFace objects. Together with existing support in Safari, font-width is now Baseline Newly available.

Browser Support

  • Chrome: not supported.
  • Edge: not supported.
  • Firefox: 155.
  • Safari: 18.4.

Source

sizes="auto" for lazy-loaded images becomes Baseline

Safari 27 adds support for the auto keyword in the sizes attribute on <img> elements that use loading="lazy".

When you specify sizes="auto" (or a sizes list beginning with auto), the browser uses the rendered layout width of the lazy-loaded image to select the most appropriate image source from srcset. With support in Chrome, Firefox, and now Safari 27, sizes="auto" is Baseline Newly available.

Browser Support

  • Chrome: 126.
  • Edge: 126.
  • Firefox: 150.
  • Safari: not supported.

Source

Async iteration and transferable ReadableStream objects become Baseline

Safari 27 includes several updates to ReadableStream:

  • Support for async iteration over ReadableStream objects using for await...of loops and ReadableStream.prototype.values().
  • Support for transferring ReadableStream objects between contexts using postMessage().
  • Support for the ReadableStream.from() static method to create a ReadableStream from an iterable or async iterable object (also supported in Firefox).

With Safari 27, async iterable streams and transferable ReadableStream objects are now Baseline Newly available.

Browser Support

  • Chrome: 124.
  • Edge: 124.
  • Firefox: 110.
  • Safari: 27.

AnimationEvent.animation and TransitionEvent.animation become Baseline

Safari 27 adds the animation property to AnimationEvent and the animation property to TransitionEvent.

This property returns the Animation object associated with the CSS animation or transition event, making it easier to inspect or control the animation directly from an event listener. With Chrome, Firefox, and Safari 27 supporting this property, it is now Baseline Newly available.

Browser Support

  • Chrome: 151.
  • Edge: 151.
  • Firefox: 152.
  • Safari: 27.

Source

The maxAge option in cookieStore.set() becomes Baseline

Safari 27 adds support for the maxAge option when setting a cookie with cookieStore.set(), letting you specify a cookie's lifetime as a duration in seconds rather than calculating an absolute expires timestamp. With support across Chrome, Firefox, and Safari 27, the maxAge option is now Baseline Newly available.

Browser Support

  • Chrome: 145.
  • Edge: 145.
  • Firefox: 148.
  • Safari: 27.

Customizable <select> in Safari 27

Safari 27 adds support for customizable <select> menus using appearance: base-select and the <selectedcontent> element, joining Chrome.

Opting a <select> and its ::picker(select) pseudo-element into appearance: base-select switches the control to a stylable base appearance, allowing rich HTML content inside <option> elements and custom styling for the button and drop-down picker.

Browser Support

  • Chrome: 135.
  • Edge: 135.
  • Firefox: behind a flag.
  • Safari: 27.

Declarative <camera> and <microphone> elements in Chrome

Chrome 153 introduces the <camera> and <microphone> HTML capability elements.

Building on the <usermedia> element, <camera> and <microphone> provide dedicated, user-activated declarative controls for requesting single-capability video or audio capture access without triggering script-initiated permission prompts on page load.

Browser Support

  • Chrome: 153.
  • Edge: 153.
  • Firefox: not supported.
  • Safari: not supported.

JavaScript Iterator and Promise additions

Chrome 153 adds Iterator.prototype.join() along with joint iteration using Iterator.zip() and Iterator.zipKeyed().

Browser Support

  • Chrome: 153.
  • Edge: 153.
  • Firefox: 148.
  • Safari Technology Preview: supported.

Source

Firefox 155 implements Promise.allKeyed() and Promise.allSettledKeyed(), which accept a dictionary object of promises and resolve to an object with the same keys.

Browser Support

  • Chrome: not supported.
  • Edge: not supported.
  • Firefox: 155.
  • Safari: not supported.

Source

WebTransport updates in Firefox 155

Firefox 155 expands its WebTransport implementation:

Browser Support

  • Chrome: 143.
  • Edge: 143.
  • Firefox: 155.
  • Safari: 26.4.

Source

Service Worker Static Routing API in Safari 27

Safari 27 adds support for the Service Worker Static Routing API through InstallEvent.addRoutes().

This API lets a service worker register declarative routing rules at install time so matching requests can bypass service worker startup and fetch directly from the network or cache, improving navigation performance.

Browser Support

  • Chrome: 123.
  • Edge: 123.
  • Firefox: not supported.
  • Safari: 27.

Source

Beta browser releases

Beta browser versions give you a preview of things that will be in the next stable version of the browser. It's a great time to test new features, or removals, that could impact your site before the world gets that release. New betas this month are Chrome 155, Chrome 156, Firefox 158, and Safari 27.2.

Chrome 155 beta includes CSS margin-trim, symbols(), and text-decoration-skip-spaces, along with JPEG XL (image/jxl) decoding support, text module imports (import ... with { type: "text" }), and Digital Credentials API issuance support.

Chrome 156 beta includes the CSS random() function, CSS corner shorthand properties, single-axis scroll containers, scroll-snap-type: pair, the image-animation property and :animated-image pseudo-class, media element state pseudo-classes (:playing, :paused, :seeking, :buffering, :stalled, :muted, :volume-locked), deferred module evaluation (import defer), and the HTML <install> element and Web Install API.

Safari 27.2 beta adds support for safe and unsafe overflow alignment when paired with normal, enables safe alignment on fixed-position boxes and ::picker(select) elements, and renames flow-tolerance to fit-tolerance for display: grid-lanes.