The EMBPERL_OBJECT_FALLBACK directive in httpd.conf allows you to set a file which will be loaded in the event that the requested file is not found. This file should be relative to the same directory as base.html.

I have found that making a special /errors/ directory is useful, because it enables that special subdirectory to define its own head.html file, init.html and so on. So, I then just put this in /notfound.html:

[- $http_headers_out{'Location'} = "/errors/"; clean_exit(); -]

See the previous section, "Handling Exits" for more on clean_exit().