The Attributes section of this document describes additional restrictions on strings in attribute values — in particular, restrictions for the following cases:
," (comma) character. Each string itself must not begin or end with any space characters, and each string itself must not contain any "
," (comma) characters.
Previous versions of HTML placed greater restrictions on the content of ID values (for example, they did not permit ID values to begin with a number).
idattribute of an element of type type anywhere in the document.
#" character, followed by a string which exactly matches the value of the
nameattribute of an element of type type anywhere in the document.
0—9, optionally prefixed with a "
e" character or "
-" character or "
Zin the date/time syntax must always be uppercase
The empty string is a valid IRI reference, so the empty string is allowed anywhere this reference lists the URL datatype as being allowed.
The empty string is a valid URL, so the empty string is allowed anywhere this reference lists the URL potentially surrounded by spaces datatype as being allowed.
Aliasfield labeled as “preferred MIME name”; or, if none of the
Aliasfields are so labeled, a case-insensitive match for a
Namefield in the registry.
53, depending on the particular year.
1*( atext / "." ) "@" ldh-str 1*( "." ldh-str )
Color keywords (for example, strings such as “red” or “green”) are not allowed.