Some of Apple's Own Mac Apps are Using HTML for UI

Jim Nielsen: Inspecting Web Views in macOS

I’m intrigued by the ability to use system colors in CSS and these web views in macOS show how engineers at Apple are leveraging non-standard CSS keywords in Webkit to mimic UIs in macOS which are otherwise built with native system APIs. It’s kind of like opening the developer tools in an Electron app, except these are native macOS apps with web views parading as system UI.