Apple expanded on the new WebKit features at a WWDC session, in which it explained to developers how they can optimize web content for viewing on Apple Watch screens.
Currently in the watchOS Mail app, rich HTML messages are rendered in a text-only format and users are prompted to view the content on another device for the full experience.
Likewise, tapping a URL link received in Mail or Messages directs Apple Watch users to their iPhone to view the web page.
However, in watchOS 5, full HTML emails are capable of being displayed on Apple Watch in cases where text-only formatting can't express the content of the message, and users can also view web page links, as well as interact with them, albeit in a limited way.
For example, turning the Digital Crown on Apple Watch scrolls the HTML content vertically, or users can drag with their finger to move up and down the page, while double-tapping zooms the content in and out, similar to iPhone.
In a Force Touch addition, a firm press on the Apple Watch screen reveals back and forward buttons to navigate through your viewing history (swiping left and right does the same thing).
Apple says it achieved the WebKit optimizations by shrinking the 320-pixel display used by the iPhone SE to fit the 156-pixel width of the Apple Watch display, and then computing the initial scale of the page so that the content width fits within the smaller screen. Basically, this allows text and images to appear smaller while preserving the overall layout of the page.
Apple notes that the optimizations are aimed at quickly consuming content, so some features like video playback and web fonts aren't currently available. However, users will be able to interact with forms within HTML content, and there's also an Safari-like Reader mode, which automatically activates on text-heavy web pages (the Reader Mode option is also accessible via Force Touch, so users can choose to read pages on their watch with extraneous content stripped out).
watchOS 5, due for release in the fall, promises several other improvements and features, including Walkie-Talkie for touch-to-talk communications with friends, automatic workout detection, and updates to the Siri watch face.