The following are banned from every error page:
<script>tags with a
<img>tags with a
srcpointing to a URL.
I am currently making an exception to the external resource rule by including a google script that adds a search box on the error pages. This is okay, as it should not cause the page to stop working and may allow the user to resolve the issue themselves.
Additionally, the error pages are render by Django, so they have a dependency on that framework. I should probably move some or all of them to be rendered solely by Nginx.
I have three different generic error pages, but hope to have more specific pages someday.