As already stated boolean attributes are attributes that are evaluated either true or false.
However, from HTML5 Spec - http://www.w3.org/TR/html5/infrastructure.html#boolean-attributes
2.5.2 Boolean attributes
A number of attributes are boolean attributes. The presence of a
boolean attribute on an element
represents the true value, and the
absence of the attribute represents
the false value.
If the attribute is present, its value must either be the empty string
or a value that is an ASCII
case-insensitive match for the
attribute's canonical name, with no
leading or trailing whitespace.
The values "true" and "false" are not allowed on boolean attributes. To
represent a false value, the attribute
has to be omitted altogether.
Note that this means that <div hidden="true">
is not allowed in HTML5.
Correct would be either <div hidden>
or <div hidden="">
or <div hidden="hidden">
or case-insensitive and single quotes/unquoted variations of any of them.
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