Mozilla documentation for touch-enabled. I haven’t found any documentation for touchscreen.
Two possibilities for figuring out whether the browser is in a touchscreen environment. I think the spec is going to go with pointer as a proxy for touchscreen browsers, so none of these will ever be well-supported. On the other hand, you never know.
touch-enabled is a Mozilla invention.
touchscreen is an old Opera invention, created at the specific request of Vodafone back in 2009 for the widget project. It may never have been supported outside the widget manager.
The curious culmination of this media query was a WebKit-based browser supporting -o-touchscreen. Don’t ask.