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.
muted" is the only token defined.
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.