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This should solve your concern. Allowed Content defines which HTML elements, attributes, styles, and classes are allowed. To get allowedContent to work, disable stylesheetparser plugin.
You can place this within your settings.py file.
CKEDITOR_CONFIGS = {'default': {
'removePlugins': 'stylesheetparser',
'allowedContent': True,},}
To learn more, you can find the documentation here.
Source:stackexchange.com