Only thing "missing" is perhaps a companion builder to emulate the C++ API with a WKWebView bundling setup for iOS.
For those reading, if Apple still disallows JIT:ed code, then a WKWebView might still be the best option in terms of pure JS simulation performance even if the WebView might steal some GPU perf.
What's the story/idea on controls (thin DOM emulation for pointerevents,keyboard,etc?), accelerometers, input-fields and fonts.
As much as I like controlling what I render, having good support for font/input-handling and system inputs is a clear plus of using web tech.