I can't find a source for this right now, I've been searching, but I remember reading something about the Messages app (I can't remember if it was iOS or MacOs/OSX) is actually a webview app! Would love it if someone could find a source proving/disproving it.
This made me curious, and I checked by attaching a debugger to Messages.
The content view with the bubbles is a web view. All the other views (sidebar, input field, etc.) are native controls.
I guess this is a sort of "best of both worlds" approach. You have all the flexibility of HTML & CSS for complex content layout, and you get performance and usability of native sidebars and text fields.
