[kovidgoyal]

No cant say I have an example, but here is a barebones viewer.py to add to your plugin. It only shows a single passed in html file, for breviry

def main(path_to_html_file, path_to_ther_html_file):
     from calibre.gui2 import Application
    app = Application([])
    w = QWebEngineView()
    w.setUrl(QUril.fromlocalFile(path_to_html_file))
    w.show()
    w.raise_()
    app.exec_()

add it to your plugin as viewer.py and call it using the run_calibre_debug() function I showed you earlier, and there you have a web engine based HTML viewer.