There appears to have been a bug in how HTTP_ACCEPT is parsed living in 
run_cgi() for eons, perhaps from the time it was written. Perhaps not 
many are using this code (I'm not either) but recent (post 3.0 Release) 
Python 3.x appear to have broken something in getallmatchingheaders() 
(which originates in Message) and I happened to stumble upon this 
condition while searching through the stdlib code.

From Line 980 of http.server

        accept = []
        for line in self.headers.getallmatchingheaders('accept'):
            if line[:1] in "\t\n\r ":
                accept.append(line.strip())
            else:
                accept = accept + line[7:].split(',')
        env['HTTP_ACCEPT'] = ','.join(accept)


line[:1] in '\t\n\r' clearly was meant to to be line[-1].

However that doesn't fix completely this chunk of code as it makes some 
assumptions about what getallmatchingheaders() delivers which aren't 
accurate. The following behaves as expected and feels safer:

        accept = []
        for line in self.headers.getallmatchingheaders('accept'):
            if line.lower().startswith("accept:"):
                line = line[7:]
            for part in line.split(','):
                part = part.strip()
                if part:
                    accept.append(part)
        env['HTTP_ACCEPT'] = ','.join(accept)


Note that post Python 3.0 release, 
http.client.HTTPMessage.getallmatchingheaders() was broken. I've 
reported this just now and proposed a fix in #5053.

I hope that someone who knows more than me on this subject takes a look at this.

Posting a patch for this so that we can get rid of the broken HTTPMessage.getallmatchingheaders() method in Issue 5053.

BTW in the original code, I think line[:1] in "\t\n\r " might have been correct. It looks like the getallmatchinheaders() method was actually meant to return continued lines separately, prefixed with whitespace. My patch is probably only appropriate for Python 3; maybe Mike’s code will work for Python 2.

New changeset b630ca7bc13ba9bdf95cd7dce0ac8e1578fb53a0 by Miss Islington (bot) in branch '3.9':
[3.9] bpo-5054: CGIHTTPRequestHandler.run_cgi() HTTP_ACCEPT improperly parsed (GH-23638) (GH-23657)
https://github.com/python/cpython/commit/b630ca7bc13ba9bdf95cd7dce0ac8e1578fb53a0

This was also resolved in 3.10 https://github.com/python/cpython/pull/23638