Qt for WebAssembly Checkboxes and Comboboxes displayed wrong

I coded a small application with QtQuick and QML to afterwards transpile it (in this case into a WebApplication) with Qt for Webassembly, which uses Emscripten internally.

When opening it, the checkboxes, and ComboBoxes look like the following:

I assume it's a HighDPI error. I've already added

QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

to my code but this changed nothing. And I've already tried changing the size.

Code:

CheckBox {
   id : contour_box
   text: qsTr("Contour Lines")
   height : 30;
}

ComboBox {
   width : source.width/3.4;
   model: [ "200ms", "500ms", "1s", "2s", "5s", "10s", "20s"]
   currentIndex: 4;
}