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I'm not sure if django-mode is derived from HTML mode or not, but assigning django-html-mode to any file extension in auto-mode-alist will cause flymake to complain in child django templates (those without an opening <html> tag).
This might be an issue with flymake, not django-mode, but I wanted to bring it to your attention.
More details here: http://stackoverflow.com/questions/9390495/making-flymake-work-with-django
If you know there is a fix for django-html-mode, hack and pull request. I don't use Emacs anymore. Vim works so much better for me.
Looks like it is a derived mode from nxml. When I find I fix, I'll do just that.