The premise is a basic misunderstanding of HTML. <div align=right> does not parse as HTML because HTML-parsers are pragmatic and obeys Postells law. It parses

The premise is a basic misunderstanding of HTML. <div align=right> does not parse as HTML because HTML-parsers are pragmatic and obeys Postells law. It parses because it is syntactically correct HTML. HTML does not require quotes around attributes if the attribute values are alphanumeric. But if you write syntactically incorrect HTML, like forgetting to match a quote (<div align="right>) you will have a problem. The parser will not show an error, but the following content may disappear. Worse, the result might be diffent in different browsers. In the one you test with, things render like you intended due to the browser-specific error recovery rules. In the browser used by most of your audience, everything on the page until the next quote (in a different tag further down the page) disappers without trace. HTML is a more complex grammar than XML which makes it harder to implement correctly. XML OTOH was designed with the KISS-principle. Clearly some characters needs to be quoted - eg. equals-signs or whitespace in an attribute value simply cannot be parsed unambiguously. Rather than having rules about which characters require quotes (does an underscore in an attribute require quotes? Does a dot?) XML just requires quotes always. The poster argues that a parser should be able to parse malformed input, since there might not be an easy way to get the producer to conform to the spec. This is a good point, but there is no simple solution, because invalid XML is by definition ambiguous. In many cases it is possible to guess what the producer intended, however it basically have to be decided on a case by case base. You can't write a parser that can parse random invalid XML and always reconstuct the data structure that the producer intended. This would require that the parser was able to read the mind of the producer, and if that was the case, XML would not be needed at all. In HTML5 there is activity to define canonical parse rules for almost any kind of malformed input. This allows more interoperable parsers, but the price is an extremely large and complex specification. Note also that this does not automatically fix mistakes. An HTML author might forget one of the quotes around an attribute value; with HTML5 there might be an official correct parse for this case. However there is no guarantee that this parse is what the author intended. There is simply no fool-proof way to fix content bugs on the client side. Postells Law have clear limits.