@saikatdas0790: Where did you read that? I'm looking at this link:
Attribute values can contain text and character references, with additional restrictions depending on whether they are unquoted attribute values, single-quoted attribute values, or double-quoted attribute values. Also, the HTML elements section of this reference describes further restrictions on the allowed values of particular attributes, and attributes must have values that conform to those restrictions.
In the the HTML syntax, attributes can be specified in four different ways:
empty attribute syntax
unquoted attribute-value syntax
single-quoted attribute-value syntax
double-quoted attribute-value syntax