Where can I download deb package "libQt5WebEngineWidgets" without full package "qt5-default" or ".run" from website?
Deb package ? You need to check with your linux distribution
I don't thing this file is included in any common Ubuntu/Debian package. You can install Qt5 from the http://www.qt.io/download-open-source/ and then copy the file libQt5WebEngineWidgets.* anywhere you want or create your deb package.