I'm very much not well versed in HTML5 interfaces myself either, but I think the process would be defining an XPanel 3.0 HTML5 touchpanel device in SIMPL (if you're using SIMPL), having the client's developers produce the interface in accordance with the HTML5 guidelines at using VSCode (and I would anticipate their having a lot of questions that you'll have to work together to figure out answers to), loading the compiled XPanel 3.0 project to the control processor and activating the SW-MOBILITY license, and then accessing the appropriate XPanel project URL via Safari on an iPad at least until a native app solution is available. But again, I haven't even played around with HTML5 enough yet myself.
-Stuart