I use this gist to concatenate stand-along html files or embed one html file in a master html file.

demote_html.py

from bs4 import BeautifulSoup

fn = r'solution1_printed.htm'
f = open(fn, 'r')
html = f.read()
f.close()
soup = BeautifulSoup(html)

body = soup.body
replaces = [
        ('h3', 'h4'),
        ('h2', 'h2'),
        ('h1', 'h2'),
        ]
for pair in replaces:
    tags = body.findAll(pair[0])
    for tag in tags:
        tag.name = pair[1]

fn = 'foo.html'
f = open(fn, 'w')
f.write(body.encode_contents())
f.close()