The Attributes section of this document describes additional restrictions on strings in attribute values — in particular, restrictions for the following cases:

• unquoted attribute values
• single-quoted attribute values
• double-quoted attribute values

• must not start with a "_" character
• must be at least one character long

• a browsing-context name
• any case-insensitive match for one of the following literal strings: "_blank", "_self", "_parent", or "_top".

• must be at least one character long
• must not contain any space characters

• must be at least one character long
• must not contain any space characters

• must be greater than zero

1. Optionally, the first character may be a "-" character.
2. One or more characters in the range "0—9".
3. Optionally, the following parts, in exactly the following order:
   1. a "." character
   2. one or more characters in the range "0—9"
4. Optionally, the following parts, in exactly the following order:
   1. a "e" character or "E" character
   2. optionally, a "-" character or "+" character
   3. One or more characters in the range "0—9".

• must be greater than zero

• the first character may not be a "-" character

• the literal letters T and Z in the date/time syntax must always be uppercase
• the date-fullyear production is instead defined as four or more digits representing a number greater than 0

• a date
• a time
• a date and time

• the literal string "any"
• two valid non-negative integers that do not have a leading "0" character and that are separated by a single "x" character.

• For documents in the HTML syntax: A case-insensitive match for any character set name for which the IANA [Character Sets] registry has a Name or Alias field labeled as “preferred MIME name”; or, if none of the Alias fields are so labeled, a case-insensitive match for a Name field in the registry.
• For documents in the XML syntax: Any case-insensitive match for the string "UTF-8".

1. The literal string "text/html;".
2. Optionally, one or more space characters.
3. The literal string "charset=".
4. A character encoding name.

• A non-negative integer.
• The string consisting of the following parts, in exactly the following order:
  1. A non-negative integer.
  2. A ";" character.
  3. One or more space characters.
  4. A case-insensitive match for the string "url=".
  5. A URL.

1. an integer representing the distance in CSS pixels from the left edge of the image to the left side of the rectangle
2. an integer representing the distance in CSS pixels from the top edge of the image to the top side of the rectangle
3. an integer, greater than the value of the first integer in this list, representing the distance in CSS pixels from the left edge of the image to the right side of the rectangle
4. an integer, greater than the value of the second integer in this list, representing the distance in CSS pixels from the top edge of the image to the bottom side of the rectangle

1. an integer representing the distance in CSS pixels from the left edge of the image to the center of the circle
2. an integer representing the distance in CSS pixels from the top edge of the image to the center of the circle
3. a non-negative integer, representing the radius of the circle, in CSS pixels

• "allow-forms"
• "allow-scripts"
• "allow-top-navigation"
• "allow-same-origin"

1. A date.
2. The literal string "T".
3. A time.

1. Four or more digits representing a number greater than 0.
2. The literal string "-".
3. Two digits, representing the month month, in the range 1 ≤ month, ≤ 12.

1. Four or more digits representing year year, where year > 0.
2. The literal string "-W".
3. Two digits, representing the week week, in the range 1 ≤ week ≤ maxweek, where maxweek is either 52 or 53, depending on the particular year.

1*( atext / "." ) "@" ldh-str 1*( "." ldh-str )

…where atext is as defined in [RFC 5322], and ldh-str is as defined in [RFC 1034].

1. A "#" character.
2. Six characters in the range 0–9, a–f, and A–F.