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HTML parser based on the WHATWG HTML specification

Project description

html5lib is a pure-python library for parsing HTML. It is designed to conform to the WHATWG HTML specification, as is implemented by all major web browsers.


Simple usage follows this pattern:

import html5lib
with open("mydocument.html", "rb") as f:
    document = html5lib.parse(f)


import html5lib
document = html5lib.parse("<p>Hello World!")

By default, the document will be an xml.etree element instance. Whenever possible, html5lib chooses the accelerated ElementTree implementation (i.e. xml.etree.cElementTree on Python 2.x).

Two other tree types are supported: xml.dom.minidom and lxml.etree. To use an alternative format, specify the name of a treebuilder:

import html5lib
with open("mydocument.html", "rb") as f:
    lxml_etree_document = html5lib.parse(f, treebuilder="lxml")

When using with urllib2 (Python 2), the charset from HTTP should be pass into html5lib as follows:

from contextlib import closing
from urllib2 import urlopen
import html5lib

with closing(urlopen("")) as f:
    document = html5lib.parse(f,"charset"))

When using with urllib.request (Python 3), the charset from HTTP should be pass into html5lib as follows:

from urllib.request import urlopen
import html5lib

with urlopen("") as f:
    document = html5lib.parse(f,

To have more control over the parser, create a parser object explicitly. For instance, to make the parser raise exceptions on parse errors, use:

import html5lib
with open("mydocument.html", "rb") as f:
    parser = html5lib.HTMLParser(strict=True)
    document = parser.parse(f)

When you’re instantiating parser objects explicitly, pass a treebuilder class as the tree keyword argument to use an alternative document format:

import html5lib
parser = html5lib.HTMLParser(tree=html5lib.getTreeBuilder("dom"))
minidom_document = parser.parse("<p>Hello World!")

More documentation is available at


html5lib works on CPython 2.6+, CPython 3.3+ and PyPy. To install it, use:

$ pip install html5lib

Optional Dependencies

The following third-party libraries may be used for additional functionality:

  • datrie can be used under CPython to improve parsing performance (though in almost all cases the improvement is marginal);

  • lxml is supported as a tree format (for both building and walking) under CPython (but not PyPy where it is known to cause segfaults);

  • genshi has a treewalker (but not builder); and

  • chardet can be used as a fallback when character encoding cannot be determined.


Please report any bugs on the issue tracker.


Unit tests require the pytest and mock libraries and can be run using the py.test command in the root directory; ordereddict is required under Python 2.6. All should pass.

Test data are contained in a separate html5lib-tests repository and included as a submodule, thus for git checkouts they must be initialized:

$ git submodule init
$ git submodule update

If you have all compatible Python implementations available on your system, you can run tests on all of them using the tox utility, which can be found on PyPI.


There’s a mailing list available for support on Google Groups, html5lib-discuss, though you may get a quicker response asking on IRC in #whatwg on

Change Log


Released on July 14, 2016

  • Added ordereddict as a mandatory dependency on Python 2.6.

  • Added lxml, genshi, datrie, charade, and all extras that will do the right thing based on the specific interpreter implementation.

  • Now requires the mock package for the testsuite.

  • Cease supporting DATrie under PyPy.

  • Remove ``PullDOM`` support, as this hasn’t ever been properly tested, doesn’t entirely work, and as far as I can tell is completely unused by anyone.

  • Move testsuite to py.test.

  • Fix #124: move to webencodings for decoding the input byte stream; this makes html5lib compliant with the Encoding Standard, and introduces a required dependency on webencodings.

  • Cease supporting Python 3.2 (in both CPython and PyPy forms).

  • Fix comments containing double-dash with lxml 3.5 and above.

  • Use scripting disabled by default (as we don’t implement scripting).

  • Fix #11, avoiding the XSS bug potentially caused by serializer allowing attribute values to be escaped out of in old browser versions, changing the quote_attr_values option on serializer to take one of three values, “always” (the old True value), “legacy” (the new option, and the new default), and “spec” (the old False value, and the old default).

  • Fix #72 by rewriting the sanitizer to apply only to treewalkers (instead of the tokenizer); as such, this will require amending all callers of it to use it via the treewalker API.

  • Drop support of charade, now that chardet is supported once more.

  • Replace the charset keyword argument on parse and related methods with a set of keyword arguments: override_encoding, transport_encoding, same_origin_parent_encoding, likely_encoding, and default_encoding.

  • Move filters._base, treebuilder._base, and treewalkers._base to .base to clarify their status as public.

  • Get rid of the sanitizer package. Merge sanitizer.sanitize into the sanitizer.htmlsanitizer module and move that to saniziter. This means anyone who used sanitizer.sanitize or sanitizer.HTMLSanitizer needs no code changes.

  • Rename treewalkers.lxmletree to .etree_lxml and treewalkers.genshistream to .genshi to have a consistent API.

  • Move a whole load of stuff (inputstream, ihatexml, trie, tokenizer, utils) to be underscore prefixed to clarify their status as private.


Released on September 10, 2015

  • Fix #195: fix the sanitizer to drop broken URLs (it threw an exception between 0.9999 and 0.999999).


Released on July 7, 2015

  • Fix #189: fix the sanitizer to allow relative URLs again (as it did prior to 0.9999/1.0b5).


Released on April 30, 2015

  • Fix #188: fix the sanitizer to not throw an exception when sanitizing bogus data URLs.


Released on April 29, 2015

  • Fix #153: Sanitizer fails to treat some attributes as URLs. Despite how this sounds, this has no known security implications. No known version of IE (5.5 to current), Firefox (3 to current), Safari (6 to current), Chrome (1 to current), or Opera (12 to current) will run any script provided in these attributes.

  • Pass error message to the ParseError exception in strict parsing mode.

  • Allow data URIs in the sanitizer, with a whitelist of content-types.

  • Add support for Python implementations that don’t support lone surrogates (read: Jython). Fixes #2.

  • Remove localization of error messages. This functionality was totally unused (and untested that everything was localizable), so we may as well follow numerous browsers in not supporting translating technical strings.

  • Expose treewalkers.pprint as a public API.

  • Add a documentEncoding property to HTML5Parser, fix #121.


Released on December 23, 2013

  • Fix #127: add work-around for CPython issue #20007: .read(0) on http.client.HTTPResponse drops the rest of the content.

  • Fix #115: lxml treewalker can now deal with fragments containing, at their root level, text nodes with non-ASCII characters on Python 2.


Released on September 10, 2013

  • No library changes from 1.0b3; released as 0.99 as pip has changed behaviour from 1.4 to avoid installing pre-release versions per PEP 440.


Released on July 24, 2013

  • Removed RecursiveTreeWalker from treewalkers._base. Any implementation using it should be moved to NonRecursiveTreeWalker, as everything bundled with html5lib has for years.

  • Fix #67 so that BufferedStream to correctly returns a bytes object, thereby fixing any case where html5lib is passed a non-seekable RawIOBase-like object.


Released on June 27, 2013

  • Removed reordering of attributes within the serializer. There is now an alphabetical_attributes option which preserves the previous behaviour through a new filter. This allows attribute order to be preserved through html5lib if the tree builder preserves order.

  • Removed dom2sax from DOM treebuilders. It has been replaced by treeadapters.sax.to_sax which is generic and supports any treewalker; it also resolves all known bugs with dom2sax.

  • Fix treewalker assertions on hitting bytes strings on Python 2. Previous to 1.0b1, treewalkers coped with mixed bytes/unicode data on Python 2; this reintroduces this prior behaviour on Python 2. Behaviour is unchanged on Python 3.


Released on May 17, 2013

  • Implementation updated to implement the HTML specification as of 5th May 2013 (SVN revision r7867).

  • Python 3.2+ supported in a single codebase using the six library.

  • Removed support for Python 2.5 and older.

  • Removed the deprecated Beautiful Soup 3 treebuilder. beautifulsoup4 can use html5lib as a parser instead. Note that since it doesn’t support namespaces, foreign content like SVG and MathML is parsed incorrectly.

  • Removed simpletree from the package. The default tree builder is now etree (using the xml.etree.cElementTree implementation if available, and xml.etree.ElementTree otherwise).

  • Removed the XHTMLSerializer as it never actually guaranteed its output was well-formed XML, and hence provided little of use.

  • Removed default DOM treebuilder, so html5lib.treebuilders.dom is no longer supported. html5lib.treebuilders.getTreeBuilder("dom") will return the default DOM treebuilder, which uses xml.dom.minidom.

  • Optional heuristic character encoding detection now based on charade for Python 2.6 - 3.3 compatibility.

  • Optional Genshi treewalker support fixed.

  • Many bugfixes, including:

    • #33: null in attribute value breaks XML AttValue;

    • #4: nested, indirect descendant, <button> causes infinite loop;

    • Google Code 215: Properly detect seekable streams;

    • Google Code 206: add support for <video preload=…>, <audio preload=…>;

    • Google Code 205: add support for <video poster=…>;

    • Google Code 202: Unicode file breaks InputStream.

  • Source code is now mostly PEP 8 compliant.

  • Test harness has been improved and now depends on nose.

  • Documentation updated and moved to


Released on February 11, 2012


Released on January 17, 2010


Released on June 12, 2008


Released on June 10, 2008


Released on October 7, 2007


Released on March 11, 2007


Released on January 8, 2007

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