The following is a list of message IDs and the related message. Many message IDs are not document here. This list is currently incomplete.

109 - Displays a warning about backward compatibility when an empty-element tag is used without a leading space for XHTML documents. This flag overrides the corresponding setting in the validator options for XHTML documents only.

110 - Displays a warning message if '>' is used in the text portion of an HTML document.

1020715002 - Performs email address syntax checking and displays a warning when an email address does not pass the check.

1020715004 - Displays a error message if the "name" attribute is used with the "a" tag and the value for the "name" attribute begins with a # character.

1020715032 - Displays a warning message if the value for the "face" attribute in a "font" or "basefont" element contains "ariel". It is a common mistake to spell the font "arial" as "ariel".

1020715033 - Displays a warning message if the HTML 4.0 category is active and the ACTION attribute is not used with the FORM element as required by HTML 4.01. You may want to disable this flag if you use JavaScript events (like "onclick") instead of the ACTION attribute. (New 20000824) (New 20020620)

1020715042 - Displays a warning when the ALIGN attribute is used without the WIDTH attribute in an HR element.

1020715049 - Displays a warning message if "alt" text is used with a value of "spacer" in an "img" element. The proper value for "alt" text should be "". (New 20020620)

1020715052 - If TYPE=TEXTAREA is used with the INPUT element, displays a warning message that it is not HTML compliant and to consider using the TEXTAREA element instead. (New 20000718)

1020715064 - Displays a error message if it appears that an invalid charset is specified in a CONTENT-TYPE attribute value in a META tag.

1020715089 - Displays an error message that HTML 4.0 requires the TYPE attribute for the script element when TYPE is missing. This flag lets you disable this message because current browsers may have problems with this attribute or do not use it.

1020715096 - Displays a warning message if the "type" attribute for a "style" tag does not equal "text/css". (New 20020520)

1020715124 - Displays a warning message if both "lang" and "xml:lang" attributes are not used when specifying a language in an XHTML 1.0 document with the "lang" attribute. (New 20020520)

1020715131 - Displays a warning if the HTML 4.0 category is active, the STYLE attribute has been used, and a default style sheet language has not been specified in the HEAD section. (New 20000712)

1020715134 - Displays a message if the "title" attribute is used and has an empty string as its value. Also displays a note about using "title" set to an empty string as a method that can be used to prevent Internet Explorer from displaying "alt" text as a tooltip. (New 20020617)

1020715135 - Displays a warning message if both "lang" and "xml:lang" attributes are not used when specifying a language in an XHTML 1.0 document with the "xml:lang" attribute. (New 20020520)

1020715146 - Displays a message that an HTML document was detected if HTML Validator detects that type of document.

1020715147 - Displays a message that an XHTML document was detected if HTML Validator detects that type of document.

1020715154 - Displays a comment message when tables may be too deeply nested and cause a problem.

1020715158 - If a PICS-Label is not present in a META tag in the HEAD section of a document, displays a message about PICS-Labels and ICRA.

1020715159 - If a Content-Type is not present in a META tag in the HEAD section of a document, displays a message about using a Content-Type META tag.

1020715161 - Displays an estimate of the document's transfer time for various transfer rates based on the document's estimated byte size. Due to inefficiencies and overhead, time is calculated assuming that one byte requires a rate of 10 bits per second (not 8 bps) to transfer that byte in one second. 1 Kbps is considered equivalent to 1000 bps (not 1024 bps). (New 20001218)

1020715187 - Displays a warning message if HTML Validator thinks that an A HREF value may be in an invalid form (such as using htp:// instead of http://). (New 20001221)

1020715188 - Displays a message that the "target" attribute is ignored if the "target" attribute is used with an "a" tag that has a "name" attribute but no "href" attribute. (New 20020521)

1020715191 - Displays a warning message if the NAME attribute is not used with the INPUT element if the INPUT element is of TYPE=TEXT, TYPE=PASSWORD, TYPE=HIDDEN, or TYPE=TEXTAREA and the INPUT element is in a form. (New 20020226)

1020715196 - Displays a message that a SMIL document was detected if HTML Validator detects that type of document. (OBSOLETE)

1020715197 - Displays a message that a RealText document was detected if HTML Validator detects that type of document. (OBSOLETE)

1020715198 - Displays a message that a WML document was detected if HTML Validator detects that type of document.

1020715199 - Displays a message that a CSE Text Inserter Configuration document was detected if HTML Validator detects that type of document.

1020715200 - Displays a message that a CFML (Colf Fusion) document was detected if HTML Validator detects that type of document.

2002071246 - Displays a message that HTML 4.01 allows the NAME attribute with a FORM element for backward compatibility and also recommends using the ID attribute to identify elements if the NAME attribute is used with a FORM element and the HTML 4.0 category is active.

2002071255 - Displays a warning message if LI is used outside of a UL, OL, DIR, or MENU list.

2002071272 - If "table" is used in "center", displays a message that different browsers may render a centered table differently. (New 20020409)

2002071286 - Displays a message if no option is explicitly selected by using the SELECTED attribute with an option in a SELECT tag. (New 20000817)

2002071528 - If FRAMESET is used, displays an accessibility message about using frames. (New 20000724)

2002071542 - Displays a Section 508 message to use client-side image maps instead of server-side if the IMG element is used with the ISMAP attribute.

2002071556 - If a description or keywords META tag is used, displays a comment message about optimizing search engine listings with SEO. (New 20020321)

2002071571 - If TABLE is used without the WIDTH attribute, displays a message that using this attribute is good style and may cause the table and page to render faster. As of v5.50, does not display this message for XHTML Basic documents because this attribute is not allowed. (New 20000922)

2002071587 - Displays a message if an HTML document is over 30,000 bytes and suggests that some people may consider the document too large.

2002071590 - Displays a comment about optimizations when the TRANSPARENCY attribute is set to 50, if the TRANSPARENCY attribute is used.

2002071593 - Displays a status comment when the validator is generating an easily parsed output file. The status comment contains information such as the number of errors, warnings, messages, tag names, comments, entities, etc.

2002071605 - Displays a message about how XML declarations may trigger "quirks" mode if an XML declaration is used in an XHTML document. (New 20020513)

2003012003 - Displays a WCAG message to use client-side image maps instead of server-side if the IMG element is used with the ISMAP attribute.

2003031400 - Displays a warning message if two or more adjacent hyphens are used in a comment. (New 20020521)

2003031401 - Displays a warning message about possible browser compatibility problems if a comment ends in "-- >" instead of "-->". (New 20020522)

2003031402 - Displays an error message for nested elements that are configured as "ignore to end tag" (such as the "script" and "style" elements). (New 20020528)

2003031700 - Displays a warning message if it appears that an absolute URL is missing the protocol (like HREF="www.htmlvalidator.com" would be missing "http://" in front of "www.htmlvalidator.com")

2003031701 - Displays a message when linking to an ID link destination that browser support is currently poor for this.

2003031702 - Displays a warning message when space characters are used in URLs. Suggests using the underscore (_) character instead or encoding spaces as "%20". (New 20000906)

2003031703 - Displays a warning message when the '%' character appears in a URL and is not followed by a two digit hexadecimal number. This may indicate improper URL encoding. (New 20000906)

2003031704 - Displays error messages if duplicate "id" or "name" ("name" for the "a" element) attribute values are used. This error message is not new but the flag to disable the message is. It is unusual and not recommended to disable this flag. (New 20020617)

2003031705 - Displays an error message for XHTML documents when a tag that would normally require a separate end tag in HTML 4 is an empty-element tag (like "a", "b", etc.). It is highly recommended that you leave this flagged checked for XHTML backward compatibility reasons. (New 20020628)

2003070200 - Displays an error message if an "input" element is of type="button" and the "value" attribute is not used. (New 20030702)

2003070800 - Displays an error message if a font family name contains a comma in a CSS property value like "font-family" or "font". (New 20030708)

2003070900 - Displays a warning message if the content of a robots or googlebot meta tag contains spaces. (New 20030709)