These error explanations were originally written by Scott Bigham, and are used here with his permission.
ID ‘FOO’ already defined”You have defined the id/name ‘FOO’ more then once. You will also get a message telling you where it was first defined. Be aware that this message may be the result of an ambiguity in the specification. While user-agents must treat values of the id attribute as case-sensitive, they must still be case-insensitive unique in the document. See Section 12.2.1 Syntax of anchor names in the HTML 4.01 Recommendation for further information.
entity end not allowed in comment”
Unterminated comment. The position indicator points to the very end of
the file, where the problem was detected; to find the beginning of the
comment, try searching for the last occurrence of
`<!--' in the document.
name start character invalid: only s and comment allowed in comment declaration”Unterminated comments again. This time, you have a case like:
<!-- This is the first unterminated comment >
<!-- This is the second one >
The validator (correctly) interprets the `--' in the
second comment as terminating the first comment, and then interprets
the text of the second comment as text between comments, which is not
allowed.
unknown declaration type ‘FOO’”
Most likely an invalid comment of the form <!invalid
comment>; the validator is (correctly) attempting to
interpret it as an SGML markup
declaration.
an attribute value must be a literal unless it contains only name characters”You have an attribute whose value needs to be in quotes. If an attribute value contains any character other than letters, digits, hyphens and periods, it must be enclosed in double quotes (single quotes would also be OK from an SGML standpoint, but some browsers don't recognize them).
character `X' not allowed in attribute specification list”The validator has found a character inside an HTML tag that isn't allowed there. This usually means you didn't put an attribute value in quotes when necessary. It also probably means that the character in question is not allowed in the value of the attribute at all, or you would have received the previous error instead. See below for more information.
You can also get in trouble by forgetting the closing quote on an attribute. For instance:
<IMG SRC="http://foo.com/fred.gif>
<IMG SRC="http://foo.com/joe.gif>
The validator will (correctly) interpret this as a SRC value of
"http://foo.com/fred.gif> <IMG SRC=", and then
choke on the following `:'. In this case, the position indicator in
the error message may be several lines below the line with the missing
quote.
length of attribute value must not exceed LITLEN less NORMSEP (1022)”This usually occurs in conjunction with the previous error. It almost always means that you've forgotten a closing quote on an attribute value. For instance, in:
<IMG SRC="fred.gif>
<!-- 50 lines of stuff -->
<IMG SRC="joe.gif">
The SRC value for the first <IMG> is the entire
fifty lines of stuff up to the next double quote, which probably
exceeds the SGML-defined length limit for
HTML string literals. Note that the position indicator in the error
message points to where the attribute value ended — in
this case, the "joe.gif" line.
element ‘FOO’ undefined”You've used an unknown tag (represented here by ‘FOO’). Make sure the DTD indicated by your DOCTYPE actually includes this element.
If the tag in question is a frame-related tag you must use the "Frameset" DTD, and if you use (deprecated) physical markup ("FONT" and friends) you must use the "Transitional" DTD.
document type does not allow element ‘FOO’ here”Straightforward, but not terribly informative. There are a lot of different errors that will generate this error message:
<TABLE><TD>blah</TABLE> will cause
this error. The trick here is that <TABLE> cannot
directly contain <TD>'s; it contains
<TR>'s which contain <TD>'s.
The above example should be
<TABLE><TR><TD>blah</TABLE>.
Similar to the previous error, but more specific: in this case, you have a ‘FOO’ element that is not contained in a ‘BAR’ element when ‘FOO’ is not allowed outside of ‘BAR’. Some of the most common causes of this error are:
DD element as a fake paragraph indent.
DD is not allowed outside of a DL element.
<OL>
<LI>Obtain the following items:
<UL>
<LI>Ham</LI>
<LI>Bread; one of:
<UL>
<LI>White</LI>
<LI>Rye</LI>
<LI>Whole wheat</LI>
</UL>
</LI> <!-- End of the "Bread" <LI> -->
<LI>Cheese</LI>
</UL>
</LI> <!-- End of the "Obtain" <LI> -->
<LI>Place ham and cheese between two slices of bread</LI>
<LI>Consume the resulting ham-and-cheese sandwich</LI>
</OL>
The </LI> end tags are optional and are only
included above to emphasize how the structure of the nested list
is arranged; if you omit them, the parser will infer their
presence in the locations shown above.
The validator has inserted a ‘BAR’ start tag where it thinks there needs to be one; it will probably complain later on that the corresponding end tag is also missing.
there is no attribute ‘FOO’ for this element (in this HTML version)”You have used an attribute with an element that is defined not to have that attribute. This is most commonly caused by using vendor-specific attributes without setting the document type appropriately.
‘FOO’ is not a member of the group specified in the declared value of this attribute”
Similar to the previous error; this time,
you're using an attribute that is defined for the element, but with a
value that isn't defined for the attribute. For instance, the Netscape
extension <IMG ALIGN=ABSMIDDLE> will cause this
error; ABSMIDDLE isn't one of the allowed values for the ALIGN
attribute of <IMG> (unless, of course, you provided
a Netscape document type).
If you're using the Netscape document type, the BORDER attribute of
<TABLE> can cause this error.
This error can also be caused, oddly enough, by missing close quotes on attribute values. For instance, given:
<IMG SRC="fred.gif> <IMG SRC="joe.gif>
The validator will (correctly) see a SRC attribute of
"fred.gif> <IMG SRC=", and then try to interpret
JOE.GIF as an attribute of <IMG>.
syntax of attribute value does not conform to declared value”Yet another attribute error; this time, the attribute in question was defined to take a numeric value, or an SGML identifier value, and you used a character that doesn't match what it was expecting.
One potentially annoying source of this error is the TD
WIDTH attribute. Whether intentionally or by oversight, the
HTML 3.2 DTD defines this attribute to have a value type of NUMBER,
which means that <TD WIDTH="50%"> is not allowed.
value of attribute ‘FOO’ invalid: "#" cannot start a name”A special case of the previous error; the attribute in question is defined to take as value an SGML name token, which must begin with a letter.
‘FOO’ is not a member of a group specified for any attribute”
Another attribute error, this time referring to an "abbreviated"
attribute. SGML allows you to omit the
name of an attribute if the attribute value is unambiguous. For
instance, <IMG ISMAP> is actually an abbreviation
of <IMG ISMAP=ISMAP>. (Technically, this also means
that e.g. <P ALIGN=CENTER> could be abbreviated to
<P CENTER>, but it is unlikely that current
browsers will get this right.) The validator has found something that
looks like an abbreviated attribute but doesn't match the value of any
attribute for this element. For instance, a typo like <IMG
ISNAP> would produce this error.
A missing close quote on a previous attribute value can also trigger
this error if the next quote it finds is followed by something that
looks like an abbreviated attribute; for instance, <IMG
SRC="fred.gif><IMG SRC="joe.gif> would produce this
error, referring to the "attribute value" JOE.GIF.
required attribute ‘FOO’ not specified”
You left off a required attribute of the indicated element. The most
common such omitted attribute is the ALT attribute of the
AREA or IMG element; browsers will typically
use these to build a menu equivalent to your client-side image map if
the user has disabled image loading, so you'll want to use a
meaningful value here.
end tag for ‘FOO’ omitted, but its declaration does not permit this”You forgot to close something. ‘FOO’ is the element you forgot to close (and the next message, `start tag was here' points to the particular instance of ‘FOO’ in question); the positional indicator points to where the validator expected you to close the element. There are a few common ways that this can happen:
<UL><LI><H4>fake font
change</H4></UL> will cause this error, since
headers aren't allowed inside <UL>'s; the
validator assumes that you meant to close the
<UL> before opening the <H4>.
<B><I>nope</B></I> will cause
this error. HTML requires that you close elements “inside-out”
— that is, the last element you opened is the first element you should
close. In this example, you'd have to close the
<I> before the <B>.
<A> tags. Yes, even <A
NAME="foo"> has to have a corresponding
</A>. Note that an empty <A
NAME> element won't be recognized by some browsers; be
sure to put at least a word or two inside the
<A>.
<DL>fake
indent</DL> will produce this error, since inside a
<DL>, the validator only expects to see
<DT>'s and <DD>'s.
<TABLE WIDTH=100%>. The
previous error should have been that the `%' was "not allowed in attribute specification list".
What's happening here is that, in trying to get past the `%', the
validator has become confused and thinks the `%' is inside
the <TABLE>...</TABLE>, thus triggering the
error message for the previous reason. This error will likely be
followed by mistaken complaints that your <TR>'s
are "not allowed here", plus a complaint
about your </TABLE> end tag. The solution is to
put the attribute value in quotes; ie. <TABLE
WIDTH="100%">.
In general, you should always explicitly close all elements and quote all attribute values.
end tag for element ‘FOO’ which is not open”
The validator found an end tag, represented here by ‘FOO’, without a
corresponding start tag. This frequently occurs in conjunction with
the previous error. For instance, given
<B><I>nope</B></I>, the validator
will insert a </I> before the
</B>, and then will find the
</I> after the </B> and will
have nothing to match it with.
A subtle variation of this is <P><H4>fake font
change</H4></P>. <H4>'s aren't
allowed inside <P>'s, but since HTML allows you to
omit the </P> end tag for paragraphs, the validator
assumes that you meant <P></P><H4>fake font
change</H4></P>, in which case the final
</P> is indeed superfluous.
This error can also be caused by incorrectly supplying an end tag for
"empty" elements like <HR>, <BR>
or <IMG>.
end tag for ‘FOO’ which is not finished”
You have a ‘FOO’ element, but you have omitted some required sub-element
of it. For instance, a TABLE with no TR's
would cause this error.
start tag for ‘FOO’ omitted, but its declaration does not permit this”
The validator expected you to start a ‘FOO’ element at the indicated
point. This probably means you've put unadorned text somewhere it
isn't allowed; for instance, <UL>fake
indent</UL> will cause this error.
unknown entity ‘FOO’”
The validator has found an entity (something like
&this;) that it doesn't recognize. There are a few
possibilities:
A reference to a URI that uses & as a separator
between parameters, such as
"http://example.org/prog?x=1&y=2".
To solve this problem, simply replace all the &'s
in attribute values with & (user agents will
convert them back before following the links.)
Another way to get around this problem is for the author of the CGI
program to allow a different value to be used between arguments,
like ';' or '|', which would allow the link to be coded as e.g.
<a href="http://example.org/prog?x=1;y=2">
this&that
for "this&that", which the validator (correctly) interprets as a
request for the entity &that;. Technically, any
non-alphanumeric character (such as a space) will suffice to
terminate the entity, but some browsers get this wrong; the safest
thing to do is to terminate all entities with a semicolon, turning
our example into this&that.
" in conjunction with the HTML 3.2
DOCTYPE. This entity was accidentally
omitted from the most recent version of the HTML 3.2 DTD. You
should be able to ignore this error safely, though if you wish, you
can replace " with the equivalent character
entity ".
non SGML character number ###”You've used an illegal character in your text. HTML uses the standard ISO8859-1 character encoding, and ISO8859-1 leaves undefined 65 character codes (0 to 31 inclusive and 127 to 159 inclusive); the validator has found one of these undefined characters in your document. The character may appear on your browser as a curly quote, or a trademark symbol, or some other fancy glyph; on a different computer, however, it will likely appear as a completely different character, or nothing at all.
Your best bet is to replace the character with the nearest equivalent ASCII character, or to use an appropriate character entity. For more information on ISO8859-1, see Alan Flavell's excellent ISO8859-1/HTML reference.
This error can also be triggered by formatting characters embedded in documents by some word processors. If you use a word processor to edit your HTML documents, be sure to use the "Save as ASCII" or similar command to save the document without formatting information.
`####' is not a valid character number”
You'll get several occurrences of this error if you use the Cougar
DTD. Cougar uses the 16-bit UCS-4 (a.k.a. "Unicode") character set
instead of the 8-bit ISO8859-1 character set used by HTML 2.0 and HTML
3.2. It also defines mnemonic entities for various Unicode characters;
for instance, the entity ™ is defined as the
character entity ™, which is the Unicode
trademark character. Unfortunately, the nsgmls executable
used by the validator does not appear to have 16-bit character support
compiled in, and so it chokes on these 16-bit character entities.
These errors are not produced by anything in your document and should not otherwise affect the validation of your document, so you can pretty much ignore them.
cannot generate system identifier for entity `HTML'”
Your DOCTYPE declaration contains a public identifier
that the validator doesn't recognize. See the discussion of DOCTYPE for an
explanation of what's happening, and what public identifiers the
validator recognizes.
Note that all of the rest of the errors you received are meaningless; the validator was unable to find a DTD to work from, and thus could not validate your document.
missing a required sub-element of ‘FOO’”The element ‘FOO’ is defined to require one or more sub-elements. One example is TR which requires one or more TD or TH elements.
start tag was here”Not an error, but rather a pointer to the start tag of the element the previous error referred to.
UTF-8 'BOM' detected and removed”The document contained an UTF-8 encoded Unicode Byte Order Mark (BOM) as the first character and we have removed it before parsing. Many XML Processors fail to recognize it (although it's perfectly valid). To be on the safe side you may want to avoid using the BOM in UTF-8 encoded documents until browsers are updated to support it.
Note that the BOM in UTF-16 encoded documents is required and handled by all conforming XML Processors.
text is not allowed here; try wrapping the text in a more descriptive container”
The document contained bare text where an element was expected.
For instance, you must wrap text in <p> if it
appears directly inside the <body>.
This error may also indicate an unquoted attribute value containing a reserved character (such as "/") which terminates the attribute. The net effect is that the rest of the attribute value appears to be plain text, outside any element, to an SGML parser.
value of attribute ‘FOO’ cannot be ‘BAR’; must be one of ‘FOO’, ‘BAR’, ‘BAZ’”An attribute was specified to contain one of a set of predefined values and you have used a value that is not in that set. The error message tells which attribute value was unknown and the possible legal values for this attribute.
character ‘FOO’ not allowed in attribute specification list possibly caused by a missing quotation mark ending a previous attribute value”A character that is illegal in the attribute list for a particular attribute was encountered. It's is likely that this is a result of a missing quote character on a previous attribute value.
duplicate specification of attribute ‘FOO’”You have specified an attribute more than once. For instance, you've used the "height" attribute on the "img" element twice on the same "img" tag.
invalid attribute value”The value of this attribute is not a legal value for attributes. For instance, the attribute cannot be the empty string. This is distinct from errors related to illegal values for a specific attribute.
an attribute specification must start with a name or name token”An attribute name (and some attribute values) must start with one of a restricted set of characters. This error usually indicates that you have failed to add a closing quotation mark on a previous attribute value (so the attribute value looks like the start of a new attribute) or have used an attribute that is not defined (usually a typo in a common attribute name).
invalid comment declaration; check your comment syntax”There is a syntax error in an SGML Comment Declaration. This may be caused by too many or too few hyphens "-" or that you have included an invalid character in the comment. The next message will be "comment declaration started here".
element ‘FOO’ not allowed here; assuming missing ‘BAR’ start-tag”The referenced element ‘FOO’ isn't allowed in the context it occurs in, but it would be if it was wrapped in a ‘BAR’ element. The Validator has assumed that you have forgotten to add a ‘BAR’ start tag and continued validation as if it was there. You should check if this is the case and insert the proper tag.