From what I read here, it means that it is not legal to write:

Regards,
Ragulf Pickaxe :-)

in the prose text they use boolean  but by that they just mean it has two
values, not present  and checked in this case. The HTML DTD defines
it to be checked=checked, but SGML rules allow that to be shortened to
checked (as no other\attribute has a declaed value of checked, so if you
just give the attribute value the attribute name is defaulted. XML
dropped these shorthands so in xhtml you have to use the full form.

> About the short way, you may be right. I do remember having used 'selected'
> without the true or false assignment in the <option> control.

yes <option selected> is short form of <option selected="selected"> and
in xhtml you have to use the full form.

David