The “form” element cannot contain any nested “form” elements. The “dfn” element cannot contain any nested “dfn” elements. The “noscript” element cannot contain any nested “noscript” elements. The “label” element cannot contain any nested “label” elements. The “address” element cannot contain any nested “address” elements. The sectioning element “blockquote” cannot appear as a descendant of the “header” element. The sectioning element “blockquote” cannot appear as a descendant of the “address” element. The sectioning element “section” cannot appear as a descendant of the “header” element. The sectioning element “section” cannot appear as a descendant of the “footer” element. The sectioning element “section” cannot appear as a descendant of the “address” element. The sectioning element “nav” cannot appear as a descendant of the “header” element. The sectioning element “nav” cannot appear as a descendant of the “footer” element. The sectioning element “nov” cannot appear as a descendant of the “address” element. The sectioning element “article” cannot appear as a descendant of the “header” element. The sectioning element “article” cannot appear as a descendant of the “footer” element. The sectioning element “article” cannot appear as a descendant of the “address” element. The sectioning element “aside” cannot appear as a descendant of the “header” element. The sectioning element “aside” cannot appear as a descendant of the “footer” element. The sectioning element “aside” cannot appear as a descendant of the “address” element. The “header” element cannot appear as a descendant of the “header” element. The “header” element cannot appear as a descendant of the “footer” element. The “header” element cannot appear as a descendant of the “address” element. The “header” element must have at least one “h1”–“h6” descendant. The “footer” element cannot appear as a descendant of the “header” element. The “footer” element cannot appear as a descendant of the “footer” element. The “footer” element cannot appear as a descendant of the “address” element. The “h1”–“h6” elements cannot appear as descendants of the “footer” element. The “h1”–“h6” elements cannot appear as descendants of the “address” element. The interactive element “a” cannot appear as a descendant of the “a” element. The interactive element “a” cannot appear as a descendant of the “button” element. The interactive element “a” cannot appear as a descendant of the “bb” element. The interactive element “datagrid” cannot appear as a descendant of the “a” element. The interactive element “datagrid” cannot appear as a descendant of the “button” element. The interactive element “datagrid” cannot appear as a descendant of the “bb” element. The interactive element “details” cannot appear as a descendant of the “a” element. The interactive element “details” cannot appear as a descendant of the “button” element. The interactive element “details” cannot appear as a descendant of the “bb” element. The interactive element “button” cannot appear as a descendant of the “a” element. The interactive element “button” cannot appear as a descendant of the “button” element. The interactive element “button” cannot appear as a descendant of the “bb” element. The interactive element “textarea” cannot appear as a descendant of the “a” element. The interactive element “textarea” cannot appear as a descendant of the “button” element. The interactive element “textarea” cannot appear as a descendant of the “bb” element. The interactive element “select” cannot appear as a descendant of the “a” element. The interactive element “select” cannot appear as a descendant of the “button” element. The interactive element “select” cannot appear as a descendant of the “bb” element. The interactive element “input” cannot appear as a descendant of the “a” element. The interactive element “input” cannot appear as a descendant of the “button” element. The interactive element “input” cannot appear as a descendant of the “bb” element. The interactive element “bb” cannot appear as a descendant of the “a” element. The interactive element “bb” cannot appear as a descendant of the “button” element. The interactive element “bb” cannot appear as a descendant of the “bb” element. The interactive element “menu type=toolbar” cannot appear as a descendant of the “a” element. The interactive element “menu type=toolbar” cannot appear as a descendant of the “button” element. The interactive element “menu type=toolbar” cannot appear as a descendant of the “bb” element. The interactive element “video controls” cannot appear as a descendant of the “a” element. The interactive element “video controls” cannot appear as a descendant of the “button” element. The interactive element “video controls” cannot appear as a descendant of the “bb” element. The interactive element “audio controls” cannot appear as a descendant of the “a” element. The interactive element “audio controls” cannot appear as a descendant of the “button” element. The interactive element “audio controls” cannot appear as a descendant of the “bb” element. The “area” element must have a “map” ancestor. The “img” element with the “ismap” attribute set must have an “a” ancestor with the “href” attribute. The value of the “value” attribute must be less than or equal to the value of the “max” attribute. The value of the “value” attribute must be less than or equal to one when the “max” attribute is absent. The value of the “min” attribute must be less than or equal to the value of the “value” attribute. The value of the “value” attribute must be greater than or equal to zero when the “min” attribute is absent. The value of the “value” attribute must be less than or equal to the value of the “max” attribute. The value of the “value” attribute must be less than or equal to one when the “max” attribute is absent. The value of the “min” attribute must be less than or equal to the value of the “max” attribute. The value of the “max” attribute must be greater than or equal to zero when the “min” attribute is absent. The value of the “min” attribute must be less than or equal to one when the “max” attribute is absent. The value of the “min” attribute must be less than or equal to the value of the “low” attribute. The value of the “low” attribute must be greater than or equal to zero when the “min” attribute is absent. The value of the “min” attribute must be less than or equal to the value of the “high” attribute. The value of the “high” attribute must be greater than or equal to zero when the “min” attribute is absent. The value of the “low” attribute must be less than or equal to the value of the “high” attribute. The value of the “high” attribute must be less than or equal to the value of the “max” attribute. The value of the “high” attribute must be less than or equal to one when the “max” attribute is absent. The value of the “low” attribute must be less than or equal to the value of the “max” attribute. The value of the “low” attribute must be less than or equal to one when the “max” attribute is absent. The value of the “min” attribute must be less than or equal to the value of the “optimum” attribute. The value of the “optimum” attribute must be greater than or equal to zero when the “min” attribute is absent. The value of the “optimum” attribute must be less than or equal to the value of the “max” attribute. The value of the “optimum” attribute must be less than or equal to one when the “max” attribute is absent. The “center” element is obsolete. The “font” element is obsolete. The “big” element is obsolete. The “s” element is obsolete. The “strike” element is obsolete. The “tt” element is obsolete. The “u” element is obsolete. The “acronym” element is obsolete. The “dir” element is obsolete. The “applet” element is obsolete. The internal character encoding declaration must be the first child of the “head” element. The “id” attribute on a “map” element must have an the same value as the “name” attribute. A “bdo” element must have an “dir” attribute. When a “table” is the first child of “datagrid”, it must not have following siblings. When a “select” element is the first child of a “datagrid” element, it must not have following siblings. When a “datalist” element is the first child of a “datagrid” element, it must not have following siblings. 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 “contextmenu” attribute must refer to a “menu” element. The “repeat-template” attribute must refer to a repetition template. The “for” attribute of the “label” element must refer to a form control. The “template” attribute of an “input” element that is of “type="add"” must refer to a repetition template. The “template” attribute of a “button” element that is of “type="add"” must refer to a repetition template. The “list” attribute of the “input” element must refer to a “datalist” element or to a “select” element. The “select” element cannot have more than one selected “option” descendant unless the “multiple” attribute is specified. An “input” element of “type="remove"” must have a repetition block or a repetition template as an ancestor. A “button” element of “type="remove"” must have a repetition block or a repetition template as an ancestor. An “input” element of “type="move-up"” must have a repetition block or a repetition template as an ancestor. A “button” element of “type="move-up"” must have a repetition block or a repetition template as an ancestor. An “input” element of “type="move-down"” must have a repetition block or a repetition template as an ancestor. A “button” element of “type="move-down"” must have a repetition block or a repetition template as an ancestor. An element with “role=option” requires “role=listbox” or “role=combobox” on the parent. An element with “role=menuitem” requires “role=menu” on the parent. An element with “role=menuitemcheckbox” requires “role=menu” on the parent. An element with “role=menuitemradio” requires “role=menu” on the parent. An element with “role=tab” requires “role=tablist” on the parent. An element with “role=treeitem” requires “role=tree” on the parent. An element with “role=listitem” requires “role=list” on the parent. An element with “role=row” requires “role=treegrid” or “role=grid” on the parent or grandparent. An element with “role=gridcell” requires “role=row” on the parent. An element must not be a child of “role=listbox” or “role=combobox” unless it has “role=option”. An element must not be a child of “role=menu” unless it has “role=menuitem”, “role=menuitemcheckbox” or “role=menuitemradio”. An element must not be a child of “role=tree” unless it has “role=treeitem”. An element must not be a child of “role=tree” unless it has “role=treeitem”. An element must not be a child of “role=radiogroup” unless it has “role=radio”. The “aria-activedescendant” attribute must refer to a descendant element. The “aria-controls” attribute must point to an element in the same document. The “aria-describedby” attribute must point to an element in the same document. The “aria-flowto” attribute must point to an element in the same document. The “aria-labelledby” attribute must point to an element in the same document. The “aria-owns” attribute must point to an element in the same document.