frabjous nailed it - the <br/> tags are all sitting uncontained, rather than being inside a block level element such as P or DIV or a heading (H1, H2, etc). At the moment, the structure is kind of like this:
Code:
<p> ...stuff inside the P element... </p>
<br/>
<p> ...stuff inside next P element... </p>
The <br/> tag needs to be inside a block level element to be valid in XHTML Strict. You could either move it inside the preceding P element, or, as frabjous suggested, get rid of the <br/> and amend the CSS formatting to increase either the bottom margin of the preceding P or the top margin of the following P.