Description of Errors

HTML5 Parse Errors in Tokenization Stage

Tag <$0> is not allowed after the end tag </html>.

The start or end tag of an element appears after the html element has been closed. The document is non-conforming.

Any content of the document other than comments must be put into the html element.

The & character must introduce a reference.

An & (U+0026 AMPERSAND) character which is not part of any reference appears in the input stream. The document is non-conforming.

The & character must be the first character of a reference:

Named entity reference
&entity-name;
where entity-name is the name of the character entity to be referenced.
Numeric character reference
&#d;
where d is the decimal representation of the code position of the character to be referenced.
Hexadecimal character reference
&#xh;
where h is the hexadecimal representation of the code position of the character to be referenced.

To represent & as a data character, use named entity reference: 

&amp;

A < character is not followed by tag name or by a ! character.

A < (U+003C LESS-THAN SIGN) character which is not part of any markup appears in the input stream.

The < character as a data character must be escaped as: 

<lt;

The decimal representation of the code position of a character must be specified after &#.

An & (U+0026 AMPERSAND) character immediately followed by a # (U+0023 NUMBER SIGN) character which is not part of any reference appears in the input stream. The document is non-conforming.

The string &# must be the first two characters NUMBER SIGN) character must be the first two characters of a reference:

Numeric character reference
&#d;
where d is the decimal representation of the code point of the character to be referenced.
Hexadecimal character reference
&#xh;
where h is the hexadecimal representation of the code point of the character to be referenced.

To represent &# as data characters, use named entity reference: 

&amp;#

The hexadecimal representation of the code position of a character must be specified after &#x.

The string &#x or &#X which is not part of any reference appears in the input stream. The document is non-conforming.

The string &#x or &#X must be the first three characters of a hexadecimal reference: 

&#xh;
where h is the hexadecimal representation of the code point of the character to be referenced.

To represent &#x as data characters, use named entity reference: 

&amp;#x

There is a -- sequence in a comment.

There is a - (U+002D HYPHEN-MINUS) character at the end of the comment or a -- sequence in the comment. The document is non-conforming.

Comments cannot contain a string --, as in XML. Unlike SGML, there cannot be more than one comments (where comment is an SGML term) in the comment declaration.

There are two attributes with same name.

There are more than one attributes with the same name in a tag. The document is non-conforming.

The motion attribute is not part of the HTML standard. Use img element with animation GIF instead.

Void element syntax (/>) cannot be used for this element.

Void element syntax (/>) must not be used for the element. The document is non-conforming.

The void element syntax can only be used for base, link, meta, hr, br, img, embed, param, area, col, and input elements.

Note that, unlike in XML, the void element syntax has no effect in HTML.

Processing instruction (<?...?>) cannot be used.

Processing instructions (<?...?>), including XML declaration (<?xml ...?>) and XML style sheet PI (<xml-stylesheet ...?>), are not allowed in the HTML syntax. The document is non-conforming.

If it is necessary to embed a processing instruction in the HTML document, you must use the XML syntax instead.

In the HTML syntax, XML declaration is not necessary.

Instead of XML style sheet, PIs, you must use the HTML link element whose rel attribute is set to stylesheet (or alternate stylesheet for an aleternate style sheet).

Web browsers will parse processing instructions as bogus comments. Some legacy Web browsers, such as IE:mac and some mobile browsers, will display processing instructions as string.

HTML5 Parse Errors in Tree Construction Stage

The $0 element cannot be inserted between head and body elements.

A start tag appears after the head element is closed but before the body element is opened. The document is non-conforming.

A DOCTYPE appears after any element or data character has been seen.

A DOCTYPE appears after any element or data character has been seen. The document is non-conforming.

The DOCTYPE must be placed before any tag, reference, or data character. Only white space characters and comments can be inserted before the DOCTYPE.

Anchor cannot be nested.

HTML a elements cannot be nested. The document is non-conforming.

In the HTML syntax, a start tag of the a implies the end tag of any opening a element.

Tag <$0> is not allowed in body.

The start or end tag of an element, which cannot be a descendant of body element, appears in the input stream while the body element has been opened. The document is non-conforming.

Tag <$0> is not allowed in table.

The start or end tag of an element, which cannot be a child of table element, appears in the input stream while the table element has been opened but no other element has been opened. The document is non-conforming.

In table, only table related elements are allowed; any other element must be contained in td or th element to form a part of the table, or caption element to create a table caption.

Data character is not allowed in table.

A data character appears in table. The document is non-conforming.

In table, only table related elements are allowed; any other element and data character must be contained in td or th element to form a part of the table, or caption element to create a table caption.

Start tag of tr element is missing.

Start tag of a tr element, which is not optional, is missing. The document is non-conforming.

In a table section, a <tr> start tag must occur before any <td> or <th> start tag. Though the HTML5 parser implies the <tr> start tag before these start tags, it must be explicitly specified.

This document does not start with the DOCTYPE.

The document does not start with a DOCTYPE. The document is non-conforming.

An HTML document must start by a DOCTYPE: 

<!DOCTYPE HTML>

Only white space characters and comments are allowed before the DOCTYPE.

Element $0 is not closed.

End tag of an element is not found before, for example, an end tag of another element appears or the end of the document. The document is non-conforming.

This document is written in an old version of HTML.

The document contains a DOCTYPE declaration that is different from HTML5 DOCTYPE (i.e. <!DOCTYPE HTML>). The document is non-conforming.

The document might or might not be conformant to some version of HTML. However, conformance to any HTML specification other than HTML5 provides for no practical convenience, since Web borwsers will parse any HTML document (roughly) as defined in HTML5.

Element $0 is not opened.

An end tag appears though no element with the same name has been opened. The document is non-conforming.

For any end tag in HTML document, there must be a corresponding start tag.

Element Content Model Errors

Data character is not allowed in this context.

A data character appears where it is not allowed in this context. The document is non-conforming.

Possible causes:

If the erred element is an inline-level element (such as a or span)

A data character cannot be a child of certain sectioning elements such as body, section, and blockquote.

Any inline-level content must be put in e.g. paragraph element such as p.

If the erred element is a block-level element (such as div or hn)

Though some elements such as div, li, and td allow either one of block-level or inline-level content is allowed. If there is a block-level content, any inline-level content must be put in e.g. paragraph element such as p.

There must be a $0 element as a child of this element.

The content model of the element is so defined that it must contain a $0 child element. The document is non-conforming.

For example:

There must be a td or th element as a child of this element.

The tr element must contain at least one td or th child element. The document is non-conforming.

This element is not allowed in this context.

An element appears where it is not allowed. The document is non-conforming.

Note: The conformance checker does not support form elements yet.

Possible causes:

If the erred element is an inline-level element (such as a or span)

An inline-level element cannot be a child of certain sectioning elements such as body, section, and blockquote.

Any inline-level content must be put in e.g. paragraph element such as p.

If the erred element is a block-level element (such as div or hn)

Though some elements such as div, li, and td allow either one of block-level or inline-level content