The <!ATTLIST keyword begins the declaration of attributes that an element may take. It is followed by the name of the element in question, a list of attribute definitions, and a closing >. Each attribute definition is a triplet that defines:

• The name of an attribute.
• The type of the attribute's value or an explicit set of possible values. Values defined explicitly by the DTD are case-insensitive.
• Whether the default value of the attribute is implicit (keyword "#IMPLIED"), in which case the default value must be supplied by the user agent (in some cases via inheritance from parent elements); always required (keyword "#REQUIRED"); or fixed to the given value (keyword "#FIXED"). Some attribute definitions explicitly specify a default value for the attribute.

4.3.4.1 DTD Entities in Attribute Definitions

Attribute definitions may also contain parameter entity references.

4.3.4.2 Boolean Attributes

Some attributes play the role of boolean variables. Their presence in the start tag of an element implies that the value of the attribute is "TRUE". Their absence implies the "FALSE" value.

Boolean attributes may legally take a single value: the name of the attribute itself.