[1] Error levels:
- (missing): An error.
- [Q@en[m]]: A violation to [['''MUST''']]-level constraint.
- [Q@en[NC]]: A namespace constraint in XML Namespace
specifications.
- [Q@en[error]]: An error.
- [Q@en[s]]: A violation to [['''SHOULD''']]-level constraint.
- [Q@en[unsupported]]: The checker does not support
the target so that it cannot be checked.

[2] Error types:
- ContentChecker:
-- [Q@en[attribute missing:[VAR@en[n]]]]: The attribute
whose name is [VAR@en[n]] is not set to the element
while the definition of the element requires it.
--- might be a [CODE@en[s]]-level error.
-- [Q@en[attribute missing:data|type]]
-- [Q@en[attribute missing:name|http-equiv]]
-- [Q@en[attribute missing:name|http-equiv|charset]]
-- [Q@en[attribute not allowed]]: The attribute is set
to an element but it is not allowed according to the
definition of it.
-- [Q@en[basehref after URI attribute]]: There is a
[CODE(HTMLe)@en[[[base]]]] element with
[CODE(HTMLa)@en[[[href]]]] attribute and the element
is after an element with URI attribute in tree order.
-- [Q@en[basetarget after hyperlink]]: There is a
[CODE(HTMLe)@en[[[base]]]] element with
[CODE(HTMLa)@en[[[target]]]] attribute and the element
is after an element that defines a hyperlink in tree order.
-- [Q@en[boolean:invalid]]: It is a boolean attribute
but the specified value is not same as the attribute name.
-- [Q@en[character not allowed]]: The character is not 
allowed where it occurs.
-- [Q@en[child element missing:[VAR@en[e]]]]: There must
be a child element [VAR@en[e]].
-- [Q@en[child element missing:atom.link.alternate]]: There must
be a child [[Atom]] [CODE(XMLe)@en[[[link]]]] element
with the [CODE(XMLa)@en[[[rel]]]] attribute set
to [CODE(XML)@en[[[alternate]]]].
-- [Q@en[child element missing:atom.link.self]]: There must
be a child [[Atom]] [CODE(XMLe)@en[[[link]]]] element
with the [CODE(XMLa)@en[[[rel]]]] attribute set
to [CODE(XML)@en[[[self]]]].
-- [Q@en[child element missing:td|th]]
-- [Q@en[coords:number:[VAR@en[n]]:[VAR@en[m]]]]: There are 
[VAR@en[m]] items in the coords value while
there should be [VAR@en[n]] items according to
the definition of it.
-- [Q@en[coords:number:even:[VAR@en[m]]]]: There are
[VAR@en[m]] items in the coords value while
there should be even number of items according to
the definition of it.
-- [Q@en[coords:number:''>''=[VAR@[n]]:[VAR@en[m]]]]: There are
[VAR@en[m]] items in the coords value while
there should be more than [VAR@en[m]] items according to
the definition of it.
-- [Q@en[coords:out of range:[VAR@en[i]]]]: The 
[VAR@en[i]][SUP@en[th]] value of the coords value is outside
of the range.
-- [Q@en[coords:syntax error]]: The specified value
is not a coords value.
-- [Q@en[dateortime:syntax error]] The specified value
is not a date or time value.
-- [Q@en[datetime:bad day]]: The day part of the date
is incorrect.
-- [Q@en[datetime:bad hour]]: The hour part of the time
is incorrect.
-- [Q@en[datetime:bad minute]]: The minute part pf the time
is incorrect.
-- [Q@en[datetime:bad month]]: The month part of the date
is incorrect.
-- [Q@en[datetime:bad second]]: The second part of the time
is incorrect.
-- [Q@en[datetime:bad timezone hour]]: The hour part
of the timezone is incorrect.
-- [Q@en[datetime:bad timezone minute]]: The minute part
of the timezone is incorrect.
-- [Q@en[datetime:syntax error]]: The datetime specification
has a syntax error.
-- [Q@en[duplicate ID]]: The [CODE(SGML)@en[[[ID]]]]
has also been specified to another element.
-- [Q@en[duplicate term]]: The term defined by this
element has also been defined by another element.
-- [Q@en[duplicate token:[VAR@en[t]]]]: It is an attribute
defined as a list of values that does not allow duplication
but there are two [VAR@en[t]]s specified.
-- [Q@en[element missing:hn]]: The element should contain
at least one [CODE(HTMLe)@en[[[h[VAR@en[n]]]]]] element
in the content, but there is no such element.
-- [Q@en[element missing:atom.[VAR@en[e]]]]: The element
should contain at leat one [CODE(XMLe)@en[[VAR@en[e]]]]
element in the [[Atom]] namespace in the content,
but there is no such element.
-- [Q@en[element not allowed]]: The element is placed where
it is not allowed according to the definition of that
element.
-- [Q@en[element not allowed:root]] The element is not allowed
as a root element, according to the knowledge of the
conformance checker.
-- [Q@en[empty attribute value]]: The attribute value
is an empty string, which is not allowed.
-- [Q@en[enumerated:invalid]]: It is an enumerated
attribute attribute and the specified value is not
a known one.
-- [Q@en[enumerated:non-conforming]]: It is an 
enumerated attribute and the specified value is one
of known values, but it is not conforming.
-- [Q@en[float:out of range]]: The specified floating point number
is outside of the range.
-- [Q@en[float:syntax error]]: The specified value is not
a floating point number.
-- [Q@en[#idref:syntax error]]: The specified value is not
a hashed ID reference.
-- [Q@en[IMT:[VAR@en[t]]]]: An Internet Media Type 
error [VAR@en[t]].
-- [Q@en[IMT:composite]]: The specified value represents
a composite type.
-- [Q@en[IMT:syntax error]]: The specified value is not
an Internet Media Type.
-- [Q@en[in HTML:xml:lang]]: A [CODE(XMLa)@en[[[xml]]:[[lang]]]]
attribute is specified in HTML document.
-- [Q@en[in XML:charset]]: An HTML [CODE(HTMLe)@en[[[meta]]]]
[CODE(HTMLa)@en[[[charset]]]] declaration is found
in an XML document.
-- [Q@en[in XML:lang]]: An HTML [CODE(HTMLa)@en[[[lang]]]]
attribute is specified in XML document.
-- [Q@en[in XML:noscript]]: An HTML [CODE(HTMLe)@en[[[noscript]]]]
element is found in XML document.
-- [Q@en[in XML:xmlns]]: An [CODE(HTMLa)@en[[[xmlns]]]]
attribute in the [CODE(DOM)@en[[[null]]]] attribute
is found in an XML document.
-- [Q@en[integer:syntax error]]: The attribute value
is not an integer.
-- [Q@en[invalid attribute value]]: The value set to the
attribute is not allowed according to the definition of
that attribute.  (If more specific error type is available,
then that one is used instead.)
-- [Q@en[keyword:invalid]]: The specified value
is not a valid keyword.
-- [Q@en[language tag:syntax error]]: The specified value
is not a language tag.
-- [Q@en[link type:bad context:[VAR@en[t]]]]:
The link type [VAR@en[t]] is not allowed for the
element it is specified to.
-- [Q@en[link type:duplicate:[VAR@en[t]]]]:
The link type [VAR@en[t]] is so defined that
there must be at most one element with that link type.
However, there are more than two elements with
that link type.
-- [Q@en[link type:non-conforming:[VAR@en[t]]]]:
The link type [VAR@en[t]] is not conforming one.
-- [Q@en[link type:proposed:[VAR@en[t]]]]:
The link type [VAR@en[t]] is proposed but not yet
accepted.
-- [Q@en[nninteger:out of range]]: The specified
non-negative integer is outside of the range.
-- [Q@en[nninteger:syntax error]]: It is not a
non-negative integer.
-- [Q@en[no document element]]: There is no document element.
-- [Q@en[no referenced map]]: There is no map with the 
specified ID.
-- [Q@en[no referenced menu]]: There is no menu with the
specified ID.
-- [Q@en[not IMT]]: The specified value does not 
represent an Internet Media Type.
-- [Q@en[ps element missing:[VAR@en[e]]]]: There should
be an [VAR@en[e]] element before this element or character.
-- [Q@en[reserved browsing context name]]: The specified
browsing context name is reserved and not conforming.
-- [Q@en[Reserved Prefixes and Namespace Names:=xml]]:
A namespace constraint error.
-- [Q@en[Reserved Prefixes and Namespace Names:xmlns:xml=]]:
A namespace constraint error.
-- [Q@en[Reserved Prefixes and Namespace Names:xmlns:xmlns=]]:
A namespace constraint error.
-- [Q@en[Reserved Prefixes and Namespace Names:<xmlns:>]]:
A namespace constraint error.
-- [Q@en[space in ID]]: There is a space character in ID.
-- [Q@en[table:[VAR@en[t]]]]:
A table error with type [VAR@en[t]].
-- [Q@en[URI::[VAR@en[t]]]]:
A URI error with type [VAR@en[t]].
-- [Q@en[URI:[VAR@en[i]]:[VAR@en[t]]]]:
A URI error with type [VAR@en[t]] at the [VAR@en[i]][SUP@en[th]]
character.
-- [Q@en[URIs:[VAR@en[n]]::[VAR@en[t]]]]:
The attribute is defined as a list of URIs.
The [VAR@en[n]][SUP@en[th]] URI has an error with type
[VAR@en[t]].
-- [Q@en[URIs:[VAR@en[n]]::[VAR@en[t]]:[VAR@en[i]]]]:
The attribute is defined as a list of URIs.
The [VAR@en[n]][SUP@en[th]] URI has an error with type
[VAR@en[t]] at the [VAR@en[i]][SUP@en[th]] character.
- HTML parser
-- [Q@en[after body:[VAR@en[e]]]]
-- [Q@en[after body:/[VAR@en[e]]]]
-- [Q@en[after body:#character]]
-- [Q@en[after frameset:[VAR@en[e]]]]
-- [Q@en[after frameset:/[VAR@en[e]]]]
-- [Q@en[after frameset:#character]]
-- [Q@en[after head:[VAR@en[e]]]]
-- [Q@en[after html:[VAR@en[e]]]]
-- [Q@en[after html:/[VAR@en[e]]]]
-- [Q@en[after html:#character]]
-- [Q@en[bare ero]]
-- [Q@en[bare etago]]
-- [Q@en[bare hcro]]
-- [Q@en[bare nero]]
-- [Q@en[bare stago]]
-- [Q@en[bogus comment]]
-- [Q@en[bogus end tag]]
-- [Q@en[C1 character reference:U+[VAR@en[HHHH]]]]
-- [Q@en[CR character reference]]
-- [Q@en[dash in comment]]
-- [Q@en[DOCTYPE in the middle]]
-- [Q@en[duplicate attribute]]
-- [Q@en[empty end tag]]
-- [Q@en[empty start tag]]
-- [Q@en[end tag attribute]]
-- [Q@en[end tag missing:[VAR@en[e]]]]
-- [Q@en[image]]
-- [Q@en[in a:a]]
-- [Q@en[in body:[VAR@en[e]]]]
-- [Q@en[in button:button]]
-- [Q@en[in CDATA:#[VAR@en[t]]]]
-- [Q@en[in form:form]]
-- [Q@en[in frameset:[VAR@en[e]]]]
-- [Q@en[in frameset:/[VAR@en[e]]]]
-- [Q@en[in frameset:#character]]
-- [Q@en[in head:head]]
-- [Q@en[in nobr:nobr]]
-- [Q@en[in noscript:[VAR@en[e]]]]
-- [Q@en[in noscript:/[VAR@en[e]]]]
-- [Q@en[in noscript:#[VAR@en[t]]]]
-- [Q@en[in RCDATA:#[VAR@en[t]]]]
-- [Q@en[in select:[VAR@en[e]]]]
-- [Q@en[in select:/[VAR@en[e]]]]
-- [Q@en[in table:[VAR@en[e]]]]
-- [Q@en[in table:/[VAR@en[e]]]]
-- [Q@en[in table:#character]]
-- [Q@en[invalid character reference:U+[VAR@en[HHHH]]]]
-- [Q@en[invalid character reference:U-[VAR@en[HHHHHHHH]]]]
-- [Q@en[isindex]]
-- [Q@en[missing start tag:[VAR@en[e]]]]
-- [Q@en[nestc]]
-- [Q@en[no DOCTYPE]]
-- [Q@en[no DOCTYPE name]]
-- [Q@en[no PUBLIC literal]]
-- [Q@en[no refc]]
-- [Q@en[no space before DOCTYPE name]]
-- [Q@en[no SYSTEM literal]]
-- [Q@en[not closed:[VAR@en[e]]]]
-- [Q@en[not first start tag]]: The [CODE(HTMLe)@en[[[html]]]]
start tag token is not the first start tag in the input stream.
-- [Q@en[not HTML5]]: A non-conforming [CODE(HTML)@en[[[DOCTYPE]]]]
token where a conformance checker may switch to
another checker for that language.
-- [Q@en[NULL]]: There is a [CODE(charname)@en[[[NULL]]]]
character in the input stream.
-- [Q@en[pio]]
-- [Q@en[string after DOCTYPE]]
-- [Q@en[string after PUBLIC]]
-- [Q@en[string after PUBLIC literal]]
-- [Q@en[string after SYSTEM]]
-- [Q@en[string after SYSTEM literal]]
-- [Q@en[unclosed attribute value]]
-- [Q@en[unclosed comment]]
-- [Q@en[unclosed DOCTYPE]]
-- [Q@en[unclosed PUBLIC literal]]
-- [Q@en[unclosed SYSTEM literal]]
-- [Q@en[unclosed tag]]
-- [Q@en[unmatched end tag:[VAR@en[e]]]]
- HTMLTable
-- [Q@en[cell overlapping:[VAR@en[x]]:[VAR@en[y]]]]
-- [Q@en[colspan creates column with no anchored cell]]
-- [Q@en[column with no anchored cell]]
-- [Q@en[rowspan expands table]]
- IMTChecker
-- [Q@en[no cell in last row]]
-- [Q@en[obsolete parameter]]
-- [Q@en[obsolete subtype]]
-- [Q@en[parameter missing:[VAR@en[attribute]]]]
-- [Q@en[private subtype]]
-- [Q@en[private type]]
-- [Q@en[unregistered subtype]]
-- [Q@en[unregistered type]]
-- [Q@en[value syntax error:[VAR@en[attribute]]]]
- URIChecker
-- [Q@en[default port]]
-- [Q@en[dot-segment]]
-- [Q@en[empty host]]
-- [Q@en[empty path]]
-- [Q@en[empty port]]
-- [Q@en[long host]]
-- [Q@en[lowercase hexadecimal digit]]
-- [Q@en[non-DNS host]]
-- [Q@en[non UTF-8 host]]
-- [Q@en[password]]
-- [Q@en[percent-encoded unreserved]]
-- [Q@en[syntax error:iri3987]]
-- [Q@en[syntax error:iriref3987]]
-- [Q@en[uppercase host]]
-- [Q@en[uppercase scheme name]]
- [CODE(perl)@en[level => unsupported]]
-- [Q@en[address format:[VAR@en[version]]]]: URI address
-- [Q@en[attribute]]
-- [Q@en[addr-spec]]
-- [Q@en[content:[VAR@en[mt]]]]
-- [Q@en[element]]
-- [Q@en[link type:[VAR@en[t]]]]
-- [Q@en[parameter:[VAR@en[attribute]]]]: IMT parameter
-- [Q@en[script:[VAR@en[mt]]]]: Conformance checking
of specified scripting language is not supported.
-- [Q@en[style:[VAR@en[mt]]]]: Conformance checking
of specified scripting language is not supported.
-- [Q@en[subtype]]: IMT subtype
-- [Q@en[type]]: IMT type
-- [Q@en[event handler]]: Validation of event handler attributes is not supported yet.
-- [Q@en[language tag]]: Validation of langauge tags is not supported yet.
-- [Q@en[media query]]: Validation of media queries is not supported yet.