правильно будет так:
[HTML]<html>
<head>
<script type="text/javascript">
function look(type)
{
param=document.getElementById(type);
if(param.style.display == "none") param.style.display = "block";
else param.style.display = "none"
}
</script>
</head>
<body>
<form>
<div id="div_1" name="div_11" style="display:block">
<h4><a href="javascript:look('div1');">Поэты:</a></h4>
<div id="div1" style="display:block">
<ul>
<li>А.С.Пушкин</li>
<li>М.Ю.Лермонтов</li>
<li>Л.Н.Толстой</li>
</ul>
</div></div>
</form>
</body>
</html>[/HTML]
или так:
[HTML]<html>
<head>
<script type="text/javascript">
function look(type)
{
param=document.getElementById(type);
param.style.display = param.style.display == "block"?"none":"block";
}
</script>
</head>
<body>
<form>
<div id="div_1" name="div_11" style="display:block">
<h4><a href="javascript:look('div1');">Поэты:</a></h4>
<div id="div1" style="display:block">
<ul>
<li>А.С.Пушкин</li>
<li>М.Ю.Лермонтов</li>
<li>Л.Н.Толстой</li>
</ul>
</div></div>
</form>
</body>
</html> [/HTML]