New to the web platform in September 2026
AI Summary
A comprehensive look at stable and beta web browser releases in September 2026, highlighting numerous features that have reached Baseline Newly available status across Chrome, Firefox, and Safari. Key updates include the progress() CSS math function, relative alpha colors via alpha(), revert-rule, <image> values in light-dark(), overflow-anchor scroll anchoring, font-width properties, sizes="auto" for lazy-loaded images, asynchronous iteration and transferable ReadableStream objects, animation event properties, cookieStore maxAge support, customizable <select> menus, and Service Worker Static Routing APIs.
Key Takeaways
- The progress() CSS function helps drive fluid sizing, opacity, or colors based on container or viewport dimensions.
- The alpha() function allows adjusting color transparency while preserving the origin color space.
- Safari 27 introduces customizable <select> menus using appearance: base-select and the <selectedcontent> element.
Why it matters
The broad inter-browser adoption of these features into Baseline means developers can now use modern CSS math functions, improved typography descriptors, efficient lazy-loading helpers, and robust stream handling in production without relying on complex polyfills or workarounds.
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