Although it feels a bit hacky to me to open the keyboard using javascript , we use Emscripten's C++ interface to create a hidden javascript text input element and set focus to that, which opens the platform keyboard.
Emscripten on Android had one other issue - the usual Emscripten input event API was not working. Nothing being typed on the native virtual keyboard was being handled like on other platforms. I found I could utilize the hidden input element that is used to pop up the keyboard, to listen for input characters and then send them to Qt.
As it is, this patch adds support for native mobile keyboard for iOS, Android and Windows on Qt for WebAssembly. (now merged into dev as 66a76a5def46d0e4a330f7130ad440c639b87cf7), too late to make it into 6.3.
Other issues on mobile are memory (as usual). Up until recently, Safari limits browser memory to much less than the other browsers.
Other areas that Qt covers for mobile devices that do not work on Qt for WebAssembly yet are sensors and bluetooth connectivity. Although there is a patch for some sensors support, it has not been merged and probably needs updating. It may not work at all, either.
Bluetooth connectivity for javascript is currently only supported on the Chrome browser and is currently experimental. While I could probably add this support to Qt WebAssembly, it would not get merged and the API might be too changey.
Any areas you find Qt WebAssembly is lacking on mobile, please report to https://bugreports.qt.io/
