so.py

#!/usr/bin/env python
# Attempt to find the cause for http://meta.stackoverflow.com/q/228587/150311
# (“Hot Network Questions” change when I hit Back button in my browser)

try:
    from http.server import *
except:
    from BaseHTTPServer import *
from datetime import datetime
import time
import gzip
try:
    from io import BytesIO
except:
    import BytesIO

class DemoHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_stuff()
    def do_HEAD(self):
        self.send_stuff(True)
    def send_stuff(self, is_HEAD=False):
        stamp = time.time()
        body = '<pre><a href="a">aaaa</a>\n'
        body += '<a href="b">bbbb</a>\n'
        body += self.date_time_string(stamp) + '\n'

        # Maybe the cache breaks on large documents? Nope.
        body += '<!-- '
        for i in range(0, 10240):
            body += 'Fill to make document larger ' + str(i)
        body += ' -->'

        if self.headers.get('Accept-Encoding'):
            body += 'Compressed\n'

            out = BytesIO()
            with gzip.GzipFile(fileobj=out, mode='w') as gzf:
                gzf.write(body.encode('UTF_8'))
            data = out.getvalue()
        else:
            data = body.encode('UTF-8')

        self.protocol_version = 'HTTP/1.1'
        try:
            self.send_response_only(200)
        except:
            self.wfile.write('HTTP/1.1 200 OK\r\n')
        # Reproduction of SO headers
        hdrs = (
            'Cache-Control: public, max-age=60',
            'Content-Type: text/html; charset=utf-8',
            'Content-Encoding: gzip',
            'Expires: ' + self.date_time_string(stamp + 60),
            'Last-Modified: ' + self.date_time_string(stamp),
            'Vary: *',
            'X-Frame-Options: SAMEORIGIN',
            'Date: ' + self.date_time_string(stamp - 1),
            'Content-Length: ' + str(len(data))
        )

        for line in hdrs:
            hdr, value = line.split(': ', 2)
            date = self.date_time_string(stamp + 3600 * 24 * 7)
            self.send_header(hdr, value)

        if not self.headers.get("Cookie"):
            self.send_header("Set-Cookie", 'foo=bar; expires=' + date + '; HttpOnly')

        self.end_headers()
        if not is_HEAD:
            self.wfile.write(data)

addr = ('', 8000)
httpd = HTTPServer(addr, DemoHandler)
httpd.serve_forever()