Bit late on parade here...
Firstly I'd comment that the inclusion of webp is a great new feature, one members have been requesting a lot recently. So with this, hidef and fit to width I'd like to say that Xara is getting back on track over the past few months.

I can only test whether the new webp inclusion serves up non webp images as an alternative by testing it on my iphone6 which has an old version of Safari. Safari is the latest big browser to support webp but the version on my iphone isn't capable afaik. This browsers loads an image. I can't find how to discover whether its a webp or jpg but I'm pretty sure it'll be the jpg version.

Here's the link to the demo site