web标准中定义id与class有什么区别吗
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<style type="text/css">
<!--
#aa {
color:red
}
.aa {
color:blue
}
-->
</style>
<div id="aa" class="aa">
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
</div>