          characters</li>
            <li>
<a href="terminology.html#must-requirement" id="must-requirement_xref33">must</a>
            not be the empty string</li>
          </ul>
<div class="example">
            <p>In the following example, the
            <code title="attr-input-value">value</code>
            attribute is given with the unquoted attribute value
            syntax:</p>
            <pre>&lt;input <em>value=yes</em>&gt;</pre>
          </div>
          <p>If the value of an attribute usi