Q about the standard

hi

for a school project im going to write a (miniature) HTML validator, but
i have a couple of questions regarding the html standard

* is this legal: <u>underlined <b>bold & underlined</u>just bold</b>  or
should it be written as :

                   <u>underlined<b>bold &underlined</b></u><b>just
bold</b>    ????


* in the 4.0 specs under block element it is writtten:

The following elements may be used as either block-level elements or
inline elements. If used as inline elements (e.g., within another inline
element or a
        P), these elements should not contain any block-level elements.

            APPLET - Java applet
            BUTTON - Button
            DEL - Deleted text
            IFRAME - Inline frame
            INS - Inserted text
            MAP - Image map
            OBJECT - Object
            SCRIPT - Client-side script


but how do i determine wether it is used as blocklevel or inline
elements ???


* The DOCTYPE element contains a link to a DTD. what is a DTD and how is
it used ??


cheers from denmark
Henrik Lynggaard