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Safari 27 Ships With Safari MCP for AI Coding Agents, Customizable Select Styling, and a Rewritten ECMAScript Module Loader

Safari 27.0 folds its Safari MCP developer server into a stable release, adds CSS-styleable select dropdowns, and rewrites the module loader in native C++.

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Overview

Apple’s WebKit team published its developer-facing rundown of Safari 27.0 on September 17, alongside Apple’s broader fall 2026 operating-system updates, according to 9to5Mac. The release folds the Safari MCP developer server into the stable browser, lets developers fully restyle the native <select> dropdown, and ships a from-scratch rewrite of Safari’s ECMAScript module loader, according to the WebKit blog.

“Our Safari release notes have never been as long as they are for this version,” the WebKit team wrote. “The number of features alone rose from 58 to 83 since the first beta in June,” according to the WebKit blog. 9to5Mac reports that Safari 27.0 “fixes nearly 850 issues, up from the 525 fixes Apple had announced just a few months ago, during WWDC.”

What We Know

  • Safari MCP graduates to stable. Safari MCP, a Model Context Protocol server that WebKit first introduced in July as a standalone tool for connecting AI coding agents to a live Safari window, is now built into the shipping browser. The original WebKit announcement explains: “By connecting your agent to a Safari browser window, your agent can emulate what your users experience, giving it the information it needs to debug more autonomously, like access to the DOM, network requests, screenshots, and console output.” The Safari 27.0 release notes describe the same capability set, saying “Safari MCP provides access to the DOM, network requests, screenshots, and console output,” according to the WebKit blog. 9to5Mac confirms the server is now shipping as part of the stable release.

  • Native <select> dropdowns can be fully restyled. Safari 27.0 lets developers restyle the standard HTML <select> element through the appearance: base-select CSS declaration, and adds targeted pseudo-elements including ::picker-icon and ::checkmark, according to the WebKit blog. The change means developers can restyle a real, accessible <select> element instead of building custom dropdown widgets out of <div> elements.

  • A new CSS stretch keyword. Safari 27.0 adds a stretch keyword for CSS sizing properties. The WebKit blog explains: “The stretch keyword tells an element to fill the available space in the relevant axis.”

  • The <model> element leaves visionOS. The HTML <model> element, for embedding 3D content the way <video> and <audio> embed media, “is now also available in Safari on iOS, iPadOS, and macOS,” according to the WebKit blog, extending a feature that had previously been limited to visionOS.

  • The ECMAScript module loader was rewritten from scratch. Safari 27.0 ships what WebKit calls “a complete standards-compliant rewrite of the ECMAScript module (ESM) loader,” according to the WebKit blog. The rewrite fixes a long-standing bug in how Safari handled top-level await. WebKit engineers explained the flaw in a separate post published September 2: “The promise for the second import shouldn’t resolve until after the first import is done evaluating, but due to a bug in the old module loader, it resolves immediately.” That ordering bug meant code could try to read another module’s exports before evaluation had finished, throwing an exception such as “Cannot access ‘someArray’ before initialization.”

What We Don’t Know

The WebKit blog post does not specify exact version numbers for the companion iOS, iPadOS, and macOS releases Safari 27.0 shipped alongside, beyond 9to5Mac’s characterization of it arriving with “Apple’s 2027 system releases.” Full setup instructions and command-line syntax for connecting third-party AI agents to Safari MCP beyond Claude and Codex were not independently verified for this article and are omitted here; developers should consult WebKit’s own documentation for exact setup steps.

Analysis

The module-loader rewrite and the Safari MCP release point in the same direction: WebKit is treating standards conformance and AI-agent tooling as adjacent priorities rather than separate tracks. A module loader implemented natively in C++ against the ECMAScript specification, rather than as self-hosted JavaScript layered on an older loading proposal, closes a category of cross-browser compatibility bugs that framework authors have had to work around. Meanwhile, folding Safari MCP directly into a stable release — rather than keeping it as a standalone add-on — signals that WebKit expects AI coding agents inspecting live DOM state, network traffic, and console output to become a routine part of the web development workflow rather than a niche one.