On performance, the new WKWebView on iOS (with Nitro JS + GPU rendering, including WebGL), and the Chromium webview on Android 5.0+ (or Crosswalk for Android 4.0+, similar features all round), radically changes the performance story. It's almost identical to the system browsers now. Facebook's comments about HTML5 were back in the dark ages when the webviews generally used horribly slow software rendering and didn't…
Incorrect at least as far as Android goes. Android's WebView literally was the system browser until Chrome was released complete with the JIT since Froyo (V8) and hardware rendering since Honeycomb. And the first several versions of Chrome on Android had much worse performance than the old WebView did.
This aside the bulk of HTML content is still software-rendered. It's software-rendered into tiles which are then GPU composited. It's a different story if you use , but at that point you aren't gaining much by paying the browser tax so why not just drop to C++ and write GLES directly and get like 10x the performance?
That aside while there have been improvements in JavaScript performance, HTML/CSS & rendering performance really has not improved much if at all over the years. Browsers are still dreadfully slow to actually produce pixels compared to any other UI toolkit.