WASM: QWidget & QWindow can't be fully transparent like desktop platforms

If I make a QWidget or QWindow fully transparent[1], I can always see a white background with one line text "XXX Reader", which blocks me from seeing through the QWidget/QWindow. Is it impossible to do such thing on WebAssembly?

 [1] For QWidget, I enabled Qt::FramelessWindowHint and Qt::WA_TranslucentBackground, for QWindow, I set the clear color to Qt::transparent. This works fine on desktop platforms.