[Andreas]

This is not possible without breaking XHTML 1.0 rules

And I still don't understand WHY you need multiple elements with the same ID ...

<style type="text/css">
.headerfont {
  font-family: Comic Sans MS;
  color:  red;
  font-weight: bold;
}
</style>

[...]

<div class="headerfont">This is one text in bold, red, Comic Sans MS</div>
<div class="headerfont">And another one</div>
<div class="headerfont">A third one</div>
<div class="headerfont">A fourth one ... does this get boring? ;)</div>