When the attribute “lang” is specified, the element must also have the attribute “lang” in the XML namespace present with the same value. When the attribute “lang” is specified, the element must also have the attribute “lang” in the XML namespace present with the same value. The “label” element cannot contain any nested “label” elements. The “form” element cannot contain any nested “form” elements. The “button” element cannot contain any nested “form” elements. The “a” element cannot contain any nested “a” elements. The “button” element cannot contain any nested “a” elements. If element “a” has an “hreflang” attribute, the “href” attribute is required as well. The “pre” element cannot contain any nested “pre” elements. The “pre” element cannot contain any nested “img” elements. The “pre” element cannot contain any nested “object” elements. The “pre” element cannot contain any nested “applet” elements. The “pre” element cannot contain any nested “big” elements. The “pre” element cannot contain any nested “small” elements. The “pre” element cannot contain any nested “sub” elements. The “pre” element cannot contain any nested “sup” elements. The “pre” element cannot contain any nested “font” elements. The “button” element cannot contain any nested “input” elements. Radio buttons and checkboxes need to have a value specified. The “button” element cannot contain any nested “select” elements. A “select” element that is not marked as multiple may not have more then one selected “option”. The “button” element cannot contain any nested “textarea” elements. The “button” element cannot contain any nested “button” elements. The “button” element cannot contain any nested “isindex” elements. The “button” element cannot contain any nested “fieldset” elements. The “button” element cannot contain any nested “iframe” elements. param element has a required name attribute. The “for” attribute of the “label” element must refer to a form control. Images contained in button element shall not have usemap atribute specified.