Ensure the language specified in either the http header or html markup (i.e. 'html lang="en"') is valid.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd"> <html lang="blah"> <head> <title>aert1.0/4.3.1 - The HTML element does not contain a valid lang attribute</title> </head> <body> </body> </html>
W3C WAI: http://www.w3.org/WAI//
HTML Techniques for WAI 1.0: http://www.w3.org/TR/WCAG10-HTML-TECHS/