nt_xref30">must</a>
      have values that conform to those restrictions.</li>
    </ul>
<p>In the <a href="documents.html#html-syntax" id="html-syntax_xref7">the HTML syntax</a>,
    attributes can be specified in four different ways:</p>
    <ol>
<li><a href="syntax.html#syntax-attr-empty" id="syntax-attr-empty_xref1">empty attribute syntax</a></li>
      <li><a href="syntax.html#syntax-attr-unquoted" id="syntax-attr-unquoted_xref2">unquoted attribute-value syntax</a></li>
      <li><a href="syntax.html#s