How to set non Drupal 404 and 403 pages for Drupal? Under Site Information we can provide custom 404 and 403 pages to Drupal, but these need to be created inside Drupal, and non Drupal pages are rejected. (such as myerror/404.html).

Contrib modules like custom error and static 404 also work inside Drupal. Any ideas on how to set an external page as 404 or 403?
If you want to catch errors not handled by Drupal, you can do this using .htaccess:

ErrorDocument 403 /error403.html
ErrorDocument 404 /error404.html
ErrorDocument 500 /error500.html


However, if you want to intercept the errors handled by Drupal, and redirect that to an standalone error page, you should be able to do this with Redirect 404.