You may get an error that says
DOCTYPE is disallowed when the feature "http://apache.org/xml/features/disallow-doctype-decl" set to true
This happens when the Critical Updates for DocType has been activated. Please De-Activate this critical update and everything should work normally.
To De-Activate it, please go to Setup and search for Critical Updates.
And on the Critical Update page, please DeActivate "Disable Custom DocTypes in Visualforce Markup".