The serializer in html5lib before 0.99999999 might allow remote attackers to conduct cross-site scripting (XSS) attacks by leveraging mishandling of the < (less than) character in attribute values.
www.openwall.com/lists/oss-security/2016/12/06/5
www.openwall.com/lists/oss-security/2016/12/08/8
www.securityfocus.com/bid/95132
github.com/html5lib/html5lib-python/commit/9b8d8eb5afbc066b7fac9390f5ec75e5e8a7cab7
github.com/html5lib/html5lib-python/issues/11
github.com/html5lib/html5lib-python/issues/12
html5lib.readthedocs.io/en/latest/changes.html#b9