, the word "may" means that
the optional feature must not be used. The term "optional" has
the same definition as "may".</dd>

<dt>Must</dt>

<dd>In this specification, the word "must" is to be interpreted
as a mandatory requirement on the implementation or on Strictly
Conforming XHTML Documents, depending upon the context. The term
"shall" has the same definition as "must".</dd>

<dt>Reserved</dt>

<dd>A value or behavior is unspecified, but it is not allowed to
be used by Conforming Documents nor to be su