in post <picayunish said:
3.2.2 Attributes
By default, SGML requires that all attribute values be delimited using
either double quotation marks (ASCII decimal 34) or single quotation
marks (ASCII decimal 39). Single quote marks can be included within the
attribute value when the value is delimited by double quote marks, and
vice versa. Authors may also use numeric character references to
represent double quotes (") and single quotes ('). For double
quotes authors can also use the character entity reference ".
In certain cases, authors may specify the value of an attribute without
any quotation marks. The attribute value may only contain letters (a-z
and A-Z), digits (0-9), hyphens (ASCII decimal 45), periods (ASCII
decimal 46), underscores (ASCII decimal 95), and colons (ASCII decimal
58). We recommend using quotation marks even when it is possible to
eliminate them.
http://www.w3.org/TR/html401/intro/sgmltut.html
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