Currently element classes have a method $addChild, which handles the adding of children plus some code to handle QML-defined default properties. That code is ugly and doesn't reflect how QtQml works. Fix this.
This project is unfortunately no longer maintained. If a new maintainer wants to step up and take care, the project is archived here: https://invent.kde.org/unmaintained/qmlweb You can just clone it in your private namespace on invent.kde.org and if you have started to work on it and fixed/implemented something get it reviewed and the project unarchived. Sorry for the inconveniences.