Output not "HTML 4.01 strict" compliant

Validation of output fails on:
- html-head-btn.html: <form>s for buttons do not contain <p> or <div> to host content,
- source: <img> in directory listing have no required alt= attribute,
- html-head.html, html-head-btn.html erroneously applies a colspan= on a <tr> tag
(As a matter-of-fact, the <table> for the variable expansion is pointless here)