The story

Today I wrote some HTML page by hand (my new homepage).
I then used this script to make the HTML code nicer:

The script

This requires the BeautifulSoup module for python

#!/usr/bin/env python

import sys
from bs4 import BeautifulSoup

def bs_file(filename):
    with open(filename, 'r') as f:
        data = f.read()
    s = BeautifulSoup(data, 'lxml')
    new_data = s.prettify()
    with open(filename, 'w') as f:
        f.write(new_data)

for f in sys.argv[1:]:
  bs_file(f)

It's pretty simple - The bs_file method reads a file, asks BS to prettify its content, and writes the result to the file again.
Do note that BS takes a lot of liberty in interpreting files - it might add tags where it decides they're missing etc.
To use the script, run it and provide the filenames you wish to prettify as arguments, e.g. /tmp/bs.py index.html