Try to clarify the optionality of giving a URL.

pages/copyvios.mako

@@ -200,7 +200,7 @@
 %>\
 <%include file="/support/header.mako" args="environ=environ, title='Copyvio Detector', add_css=('copyvios.css',), add_js=('copyvios.js',)"/>
             <h1>Copyvio Detector</h1>
-            <p>This tool attempts to detect <a href="//en.wikipedia.org/wiki/WP:COPYVIO">copyright violations</a> in articles. Simply give the title of the page you want to check and hit Submit. The tool will then search for its content elsewhere on the web and display a report if a similar webpage is found. If you provide a URL, it will not query any search engines and instead display a report comparing the article to that particular webpage, like the <a href="//toolserver.org/~dcoetzee/duplicationdetector/">Duplication Detector</a>. Check out the <a href="//en.wikipedia.org/wiki/User:EarwigBot/Copyvios/FAQ">FAQ</a> for more information and technical details.</p>
+            <p>This tool attempts to detect <a href="//en.wikipedia.org/wiki/WP:COPYVIO">copyright violations</a> in articles. Simply give the title of the page you want to check and hit Submit. The tool will then search for its content elsewhere on the web and display a report if a similar webpage is found. If you also provide a URL, it will not query any search engines and instead display a report comparing the article to that particular webpage, like the <a href="//toolserver.org/~dcoetzee/duplicationdetector/">Duplication Detector</a>. Check out the <a href="//en.wikipedia.org/wiki/User:EarwigBot/Copyvios/FAQ">FAQ</a> for more information and technical details.</p>
             <form action="${environ['PATH_INFO']}" method="get">
                 <table>
                     <tr>
@@ -223,7 +223,7 @@
                         % endif
                     </tr>
                     <tr>
-                        <td>URL:</td>
+                        <td>URL (optional):</td>
                         % if url:
                             <td><input type="text" name="url" size="120" value="${url | h}" /></td>
                         % else: