WASM: build doesn't seem to support high-dpi

When building an application with the webassembly target, the rendering on high DPI screens is quite blury by default - while the build of the same application with  classical targets like desktop or ios are rendered with sharp edges.

I tried to add some scaling magic to the HTML canvas, but that didn't help. From the view of portability, shouldn't `QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);` be sufficient? What has to be done to make the application look crispy in the web view, too?