That’s an XHTML doctype that requires that the page be served as XHTML and not as HTML. Self closing tags in XHTML do not require a space before the /> and with the transitional doctype that could also be served as HTML the space was only required for Netscape 4.
A better alternative now if you actually are using XHTML is to use XHTML 5 which doesn’t have or need a doctype (since XHTML 5 and HTML 5 do not follow the SGML standard and the doctype is an SGML tag apart from in HTML 5 where it was added as an HTML only tag).