    (including in
      <a href="syntax.html#syntax-comments" id="syntax-comments_xref5">comments</a>)
      and
      <a href="syntax.html#syntax-attribute-value" id="syntax-attribute-value_xref5">attribute values</a>
      <a href="terminology.html#must-requirement" id="must-requirement_xref37">must</a>
      consist of Unicode characters, with the following
      restrictions:</p>
    <ul>
<li>
<a href="terminology.html#must-requirement" id="must-requirement_xref38">must</a>
      not contain U+0000 char