The W3C’s online markup checker doesn’t throw errors on this yet, but URI’s are not supposed to have spaces in them, at least not in XHTML Strict. So code like this:
onclick="window.open(this.href); return false;"
onkeypress="window.open(this.href); return false;">...</a>
<a href="<?php bloginfo('stylesheet_directory'); ?>/email.php?p=<?php the_ID(); ?>" onclick="window.open(this.href, 'etc.'); return false;" onkeypress="window.open(this.href, 'etc.'); return false;">...</a>
for both the E-mail and Print functions (with appropriate changes for the latter) and volia, the blog is validating again/for real!