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 month
• a date
• a yearless date which must consist of the following parts in exactly the following order:
  1. a valid date-month as defined in [RFC 3339]
  2. The literal string "-".
  3. a valid date-mday as defined in [RFC 3339]

  Example:

  11-12

• a time
• a local date and time
• a valid time-offset as defined in [RFC 3339]

  Examples:

  Z
  +0000
  +00:00
  -0800
  -08:00

• a date and time
• a week
• a valid date-fullyear as defined in [RFC 3339], with the additional qualification it must be four or more digits representing a number greater than 0

  Examples:

  2011
  0001

• a valid duration string as defined in the [HTML5] specification

  Examples:

  PT4H18M3S
  4h 18m 3s

• 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.

1. The literal string "text/html;".
2. Optionally, one or more space characters.
3. The literal string "charset=".
4. One of the following:
   • For documents in the HTML syntax: A character encoding name.
   • For documents in the XML syntax: Any case-insensitive match for the string "UTF-8".

• 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.

An unordered set of unique space-separated tokens, each of which is a case-insensitive match for one of the following:

copy

Indicates that dropping an accepted item on the element will result in a copy of the dragged data.

move

Indicates that dropping an accepted item on the element will result in the dragged data being moved to the new location.

link

Indicates that dropping an accepted item on the element will result in a link to the original data.

Any string with three characters or more, beginning with the literal string "string:".

Indicates that Plain Unicode string items, of the type indicated by the part of of the keyword after the "string:" string, can be dropped on this element.

Any string with three characters or more, beginning with the literal string "file:".

Indicates that File items, of the type indicated by the part of of the keyword after the "file:" string, can be dropped on this element.

The value must not have more than one of the three tokens "copy", "move", or "link". If none are specified, the element represents a copy dropzone.

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].

That is, any string which matches the following regular expression:

/^[a-zA-Z0-9.!#$%&’*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$/

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