The errors I get are...
# Error Line 201, Column 4: end tag for "ul" which is not finished.
Most likely, you nested tags and closed them in the wrong order. For example <p><em>...</p> is not acceptable, as <em> must be closed before <p>. Acceptable nesting is: <p><em>...</em></p>
Another possibility is that you used an element which requires a child element that you did not include. Hence the parent element is "not finished", not complete. For instance, in HTML the <head> element must contain a <title> child element, lists (ul, ol, dl) require list items (li, or dt, dd), and so on.
I think it has to do with the fact that the tags are nested (and are unclosed) which isn't allowed for XHTML 1.0 Transitional from what I understand. The instructions said something about using HTML 4.01 Transitional (with a loose.dtd) but that doesn't validate with the rest of the page.
The version I have probably is an older version as I wasn't even aware there is a newer one.