Understanding HTML, XML and XHTML

Understanding HTML, XML and XHTML: “[T]he vast majority of supposedly XHTML documents on the internet are served as text/​html. Which means they are not XHTML at all, but actually invalid HTML that’s getting by on the error handling of HTML parsers. All those ​‘Valid XHTML 1.0!’ links on the web are really saying ​‘Invalid HTML 4.01!’ ”