Home » Html » Validate (X)HTML in Python

Validate (X)HTML in Python

Posted by: admin November 30, 2017 Leave a comment


What’s the best way to go about validating that a document follows some version of HTML (prefereably that I can specify)? I’d like to be able to know where the failures occur, as in a web-based validator, except in a native Python app.


XHTML is easy, use lxml.

HTML is harder, since there’s traditionally not been as much interest in validation among the HTML crowd (run StackOverflow itself through a validator, yikes). The easiest solution would be to execute external applications such as nsgmls or OpenJade, and then parse their output.


PyTidyLib is a nice python binding for HTML Tidy. Their example:

from tidylib import tidy_document
document, errors = tidy_document('''<p>f&otilde;o <img src="bar.jpg">''',
print document
print errors

Moreover it’s compatible with both legacy HTML Tidy and the new tidy-html5.


I think the most elegant way it to invoke the W3C Validation Service at


programmatically. Few people know that you do not have to screen-scrape the results in order to get the results, because the service returns non-standard HTTP header paramaters

X-W3C-Validator-Recursion: 1
X-W3C-Validator-Status: Invalid (or Valid)
X-W3C-Validator-Errors: 6
X-W3C-Validator-Warnings: 0

for indicating the validity and the number of errors and warnings.

For instance, the command line

curl -I "http://validator.w3.org/check?uri=http%3A%2F%2Fwww.stalsoft.com"


HTTP/1.1 200 OK
Date: Wed, 09 May 2012 15:23:58 GMT
Server: Apache/2.2.9 (Debian) mod_python/3.3.1 Python/2.5.2
Content-Language: en
X-W3C-Validator-Recursion: 1
X-W3C-Validator-Status: Invalid
X-W3C-Validator-Errors: 6
X-W3C-Validator-Warnings: 0
Content-Type: text/html; charset=UTF-8
Vary: Accept-Encoding
Connection: close

Thus, you can elegantly invoke the W3C Validation Service and extract the results from the HTTP header:

# Programmatic XHTML Validations in Python
# Martin Hepp and Alex Stolz
# [email protected] / [email protected]

import urllib
import urllib2

URL = "http://validator.w3.org/check?uri=%s"
SITE_URL = "http://www.heppnetz.de"

# pattern for HEAD request taken from 
# http://stackoverflow.com/questions/4421170/python-head-request-with-urllib2

request = urllib2.Request(URL % urllib.quote(SITE_URL))
request.get_method = lambda : 'HEAD'
response = urllib2.urlopen(request)

valid = response.info().getheader('X-W3C-Validator-Status')
if valid == "Valid":
    valid = True
    valid = False
errors = int(response.info().getheader('X-W3C-Validator-Errors'))
warnings = int(response.info().getheader('X-W3C-Validator-Warnings'))

print "Valid markup: %s (Errors: %i, Warnings: %i) " % (valid, errors, warnings)


You can decide to install the HTML validator locally and create a client to request the validation.

Here I had made a program to validate a list of urls in a txt file. I was just checking the HEAD to get the validation status, but if you do a GET you would get the full results. Look at the API of the validator, there are plenty of options for it.

import httplib2
import time

h = httplib2.Http(".cache")

f = open("urllistfile.txt", "r")
urllist = f.readlines()

for url in urllist:
   # wait 10 seconds before the next request - be nice with the validator
   resp= {}
   url = url.strip()
   urlrequest = "http://qa-dev.w3.org/wmvs/HEAD/check?doctype=HTML5&uri="+url
      resp, content = h.request(urlrequest, "HEAD")
      if resp['x-w3c-validator-status'] == "Abort":
         print url, "FAIL"
         print url, resp['x-w3c-validator-status'], resp['x-w3c-validator-errors'], resp['x-w3c-validator-warnings']


Try tidylib. You can get some really basic bindings as part of the elementtidy module (builds elementtrees from HTML documents). http://effbot.org/downloads/#elementtidy

>>> import _elementtidy
>>> xhtml, log = _elementtidy.fixup("<html></html>")
>>> print log
line 1 column 1 - Warning: missing <!DOCTYPE> declaration
line 1 column 7 - Warning: discarding unexpected </html>
line 1 column 14 - Warning: inserting missing 'title' element

Parsing the log should give you pretty much everything you need.


I think that HTML tidy will do what you want. There is a Python binding for it.


In my case the python W3C/HTML validation packages did not work pip search w3c (as of sept 2016).

I solved this with

$ pip install requests

$ python
Python 2.7.12 (default, Jun 29 2016, 12:46:54)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> r = requests.post('https://validator.w3.org/nu/', 
...                    data=file('index.html', 'rb').read(), 
...                    params={'out': 'json'}, 
...                    headers={'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2272.101 Safari/537.36', 
...                    'Content-Type': 'text/html; charset=UTF-8'})

>>> r.text
>>> u'{"messages":[{"type":"info", ...

>>> r.json()
>>> {u'messages': [{u'lastColumn': 59, ...

More documentation here python requests, W3C Validator API


This is a very basic html validator based on lxml’s HTMLParser. It does not require any internet connection.

_html_parser = None
def validate_html(html):
    global _html_parser
    from lxml import etree
    from StringIO import StringIO
    if not _html_parser:
        _html_parser = etree.HTMLParser(recover = False)
    return etree.parse(StringIO(html), _html_parser)

Note that this will not check for closing tags, so for example, the following will pass:

validate_html("<a href='example.com'>foo</a>")

However, the following wont:

validate_html("<a href='example.com'>foo</a")