1. Install a DXP instance
2. Start it without installing the theme
3. Open IE11/Edge and its console
The mentioned error doesn't occur.
4. Deploy a custom theme.
The error is clearly caused by the incorrect HTML output that is generated. Because it is not valid and confusing, it can cause errors on in more strict browser engines (like Trident/EdgeHTML):
Also an error occurs in the DXP HTML generation:
Please check the attached testtest_-src.zip_ and testtest_-1.0.0.jar_ files.
The following commands have been used for those: