The macro definition is done with a div tag. Due to the missing talmit-tag, the <div> is rendered in a definition list where no div is allowed. See marked change below. Now the XHTML Validator does not complain any more.

<!-- macro used by the portlets to compile a list of recent additions -->
<div metal:define-macro="portletRecent" talmit-tag="">
     <tal:x tal:condition="maxItems|nothing"><tal:x tal:define="global maxResults maxItems"/>
     </tal:x><tal:x tal:condition="not: maxItems|nothing"><tal:x tal:define="global maxResults python:0"/>
     </tal:x><tal:block tal:define="maxResults python:test(maxResults==0,here.simpleblog_tool.getMaxItemsInPortlet(),maxResults);
                                    recent python:startpoint.getEntries(maxResults=maxResults,skipOnTop=1, join=1);
                                    global showIcons startpoint/getShowIcons;"
     talmit-tag=""><tal:toggle tal:define="global toggle python:1"/>
    <tal:entries tal:repeat="entry recent">
        <tal:start tal:condition="python:inBlog and not solo">
        <dd class="" tal:define="oddrow repeat/entry/odd"
                     tal:condition="repeat/entry/start"
                     tal:attributes="class python:test(toggle, 'portletItem odd','portletItem even')">
            <tal:frontpage>
                <strong i18n:translate="recent_additions">Recent entries:</strong>
            </tal:frontpage>
        </dd></tal:start>
        <dd class=""
            tal:define="oddrow repeat/entry/odd;
                        item_type entry/portal_type;
                        item_type_class python:'contenttype-' + normalizeString(item_type);"
            tal:attributes="class python:test(toggle, 'portletItem even','portletItem odd')">
            <a href="#"
               tal:attributes="href entry/absolute_url;"
                               title="entry">
               <span tal:attributes="class python:showIcons and 'simpleBlogPortletIcons '+item_type_class or ''" tal:content="entry/title_or_id"/>
            </a><tal:toggle tal:define="global toggle python:test(toggle==1,0,1)"/>
        </dd>
        <dd class=""
            tal:define="oddrow repeat/entry/odd;
                        solo solo|python:0;
                        className python:test(solo,'portletFooter', 'portletItem')"
            tal:condition="repeat/entry/end"
            tal:attributes="class python:test(toggle, className + ' even', className + ' odd')">
            <a href="#" tal:attributes="href string:${startpoint/absolute_url}/SimpleBlogFullSearch"
                           title="more..."
                           i18n:attributes="title box_morelink"
                           i18n:translate="box_morelink"
                           i18n:domain="plone">More...</a>
        </dd></tal:entries>
    <tal:no-items tal:condition="not: recent">
        <dd class="portletItem even">
            <strong i18n:translate="recent_additions">Recent additions:</strong>
        </dd>
        <dd class="portletItem odd">
            <span i18n:translate="no_published_blog_entries">No blog entries are published.</span>
            <tal:toggle tal:define="global toggle python:0"/>
        </dd></tal:no-items></tal:block>
</div>

Hmm. No plain HTML possible here? OK. The strange smilies above mean < t a l : o m i t - t a g = " " > . Just in case it was not obvious. And now a real smilie