Safari 27 beta previews customizable selects and broader web-platform updates

WebKit previewed Safari 27 beta on June 8 with customizable <select> controls, scroll-anchoring controls, JavaScript Promise Integration for WebAssembly, and a wide set of CSS and Web API changes. The WWDC26 overview is a beta announcement, so it describes a testing target rather than a final production release.

Native controls become more designable

Customizable Select expands how authors can structure and style selection interfaces while retaining a native element. The attraction is fewer bespoke combobox implementations, which often reproduce keyboard navigation, focus management, and form semantics incompletely. The migration risk is assuming that visual customization automatically preserves every accessibility and input behavior.

Safari 27 beta also includes explicit controls for scroll anchoring, the browser behavior that tries to keep a user’s viewport stable when content above it changes. That can help feeds and asynchronously populated pages, but developers need to distinguish useful stabilization from cases where an application intentionally moves content.

JSPI addresses a different boundary: WebAssembly code written around synchronous operations can interoperate with JavaScript Promise-based APIs without an application manually restructuring the whole call stack. This is especially relevant to ports of existing native applications.

Use the beta to find assumptions

A useful test plan begins with native semantics. Exercise customized selects with keyboard, touch, zoom, high-contrast settings, validation errors, and form reset. For scroll anchoring, test image loading, prepend operations, virtualized lists, and history restoration. For WebAssembly, test rejection and cancellation paths as carefully as successful suspension.

Because the source covers a beta, teams should avoid freezing production code around unverified details. Add targeted experiments to Safari Technology Preview or beta CI, report reduced failures to WebKit, and keep feature detection in place. The value of the preview is early feedback: it gives component and tool authors time to find parsing, styling, and lifecycle assumptions before Safari 27 reaches stable users.