You need to install XMLParserHTML from the Config(Pharo4) or Catalog(Pharo5) browser.
Disabling validation like Vincent said isn't enough because non-validating xml parsers still expect documents to be wellformed (which includes proper nesting and closing of tags). Validation is just a set of additional constraints like checking against a DTD if present. XMLParserHTML will load SAX and DOM parsers that accept messy HTML, but they were kind of an afterthought so I don't promote them to people already using something else (like Soup or Todd's parser) that are happy with it. Their advantage is speed (the fastest HTML parsers for Pharo or Squeak by far) and integration with XMLParser and related libs.
