Safari 27.1 previews fold APIs on iPhone Duo, but not full-width pages
Apple's WebKit team details Duo viewports, flagged fold APIs, and an unsolved gap beside Safari's side controls.
Apps · Macfolio · 25 October 2026
Apple's WebKit team published "WebKit Features for Safari 27.1" on October 23, 2026, the day iPhone Duo went on sale. Safari 27.1 ships only on the Duo for now, since iOS 27.1 is a Duo-only release according to Apple's security releases page. The headline for web teams is that nothing breaks: it is the same WebKit engine as on every other iPhone.
What changes in Safari 27.1
- PDFs on the web open as two pages side by side when the Duo is held partially folded like a book, and switch back to one page when it is flat or closed.
- The Device Posture API and the Viewport Segments API are available as a developer preview, off by default. Turn them on under Settings, Apps, Safari, Advanced, Feature Flags.
- Device Posture reports "folded" or "continuous", while Viewport Segments exposes the areas on each side of the fold to CSS media queries and JavaScript.
Apple also lists the viewport sizes: widths from 382 to 951 CSS pixels and heights from 368 to 902, depending on which display is in use, its orientation, and whether two windows share the inner screen. Apple says Xcode 27.1 or later includes Device Hub, which can run an iPhone Duo simulator.
The unsolved part: the side strip
The outer display is wider and shorter than other iPhone screens, so iOS moves Safari's controls to the side. In Safari 27.1, pages do not extend behind those controls; Safari fills the strip with the page's background color. Apple says it wants to let designers use that space but is wary of the obvious fix, the existing "viewport-fit=cover" setting, because many sites fail to inset content on all four edges. It is investigating alternatives and invites feedback.
Who should act
If you build or maintain web apps used on iPhones, test in the Duo simulator now and avoid device sniffing, as Apple advises. Treat the flagged APIs as experiments, since Apple says they may still change.
This does not mean sites need a Duo-specific redesign; responsive layouts should already work.
Our take: the honest admission that the edge problem is unsolved is useful, so wait for Apple's final approach before adding Duo-only layout code to production sites.
Caveat: this summary relies on Apple's post, not hands-on testing.