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WebKit Says Safari 27.1 on iPhone Duo Previews Device Posture and Viewport Segments APIs Behind Feature Flags

WebKit's Safari 27.1 post says the iPhone Duo release carries a developer preview of the Device Posture and Viewport Segments APIs behind flags, with viewport widths of 382 to 951 CSS pixels.

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Overview

WebKit published its WebKit Features for Safari 27.1 post on October 23, 2026, written by Jen Simmons. The post says “Today iPhone Duo arrives, and with it, Safari 27.1.” For web developers, it says Safari 27.1 on iPhone Duo includes a preview of the Device Posture API and the Viewport Segments API, available behind feature flags. Apple’s foldable was previously covered when App Store submissions opened.

What We Know

No new engine. According to WebKit, the “big news” is no news: the device runs Safari with “the same WebKit engine and the same web technologies as every other iPhone.” The post adds that websites and web apps work on iPhone Duo just like they do everywhere else.

One user-facing change. Per the same post, when someone holds the phone partially folded like a book while viewing a PDF on the web, Safari 27.1 shows the PDF as two pages side by side. It switches back to one page when the screen is opened flat or closed fully.

Viewport sizes. WebKit says viewport width in CSS pixels can range from 382 to 951 in Safari 27.1, and height from 368 to 902. Its table lists the outer display in portrait at 382 wide with a small viewport height (SVH) of 580 and a large viewport height (LVH) of 618, and the inner display in landscape with one tab at 951 wide, 585 SVH and 620 LVH. With two windows side by side on the inner display, each is 385 wide. The post advises avoiding user-agent sniffing in favor of feature detection.

Testing without hardware. WebKit says developers can download Xcode 27.1 or later to get Device Hub, where a simulator for iPhone Duo can be created.

The Two Previewed APIs

WebKit says both APIs are a developer preview. To enable them, go to Settings > Apps > Safari > Advanced > Feature Flags and turn on “Device Posture API” and “Viewport Segments API.”

Device Posture API. WebKit says it reports whether the screen is partially folded through the device-posture media query and navigator.devicePosture. Both report one of two postures: folded, when the device is partially folded in a book or laptop pose, and continuous, when the screen is flat, which includes fully open and fully closed displays. MDN marks the API as experimental and notes that current browser developer tools emulate only fully open or closed devices, so they always return continuous.

Viewport Segments API. WebKit says Safari 27.1 supports the horizontal-viewport-segments and vertical-viewport-segments media queries, which match the number of segments laid out side by side or stacked. It also adds six environment variables, from viewport-segment-width to viewport-segment-left, each taking the segment’s column and then row index. In JavaScript, window.viewport.segments returns an array of DOMRect objects, one per segment.

What We Don’t Know

  • WebKit says that because both APIs are behind flags, “there may still be time to evolve how they work.” The post gives no date for shipping them unflagged.
  • The post says web content in Safari 27.1 does not currently extend to fill the space behind the controls on the outer display. WebKit says it is weighing options, including viewport-fit="cover", and is investigating whether there is “a better way” to do it. It has not announced a decision.