HTML5 apps using Xamarin?

I know how Xamarin works and have been working on it for quite some time but one of my clients is adamant of have HTML5 apps using Xamarin. I know Xamarin supports only native UIs, but am i missing anything?

Basically my client needs to have his backend and frontend both in .NET but he needs UI in HTML5 so that in future he wants to change UI, its easy and maintainable. I've tried to convince him but he is pretty strong on having HTML5 UIs.

Am i missing anything here?

Thank you, Rupreet