I already had a Unity web (HTML+WebGL+webassembly) game with pay-what-you-want downloads but just to be sure it works I tried setting it to Paid and the page does explicitly say the web version is still freely available.
https://technicat.itch.io/learnunity
But I've seen anecdotal reports and in my own experience web games generate less contributions than downloadable games-- I don't know ifthat applies to this scenario, but if it does, an alternative is to have one page just for the web demo and one for the paid download and have them link to each other.