If you see tags like <br /> in the source code of your webpage, it's because the output of Nucleus (except the things defined by templates and skins, of course) is compliant to the XHTML 1.0 standard from the W3C, which is the successor of HTML 4. This way, Nucleus is ready for the future of the web. As far as I know, this XHTML support does not cause any trouble with older browsers and is correctly interpreted.

What this means, is that you can perfectly create an XHTML-compliant site by using correct skins and templates. The default Nucleus skin is XHTML-compliant, but uses the "HTML 4 Loose" doctype. This way, users not knowing XHTML can not create documents with a false XHTML doctype.