In older QGIS versions, an incompatibility between the modern Qt "WebEngine" classes and QGIS' 3D framework prevented QGIS plugins from using any WebEngine widgets. This was a significant shortcoming, as the older Qt "WebKit" widgets were very limited, slow, and did not support many modern JavaScript essentials.
In 3.36, we've reworked how QGIS 3D is initialised to remove this incompatibility. Scripts and plugins can now safely use Qt's WebEngine classes without issue!