<html lang="ja">
<meta charset="utf-8">
<script>
window.onload=function() {
if (!sessionStorage.getItem("次の表示"))
{sessionStorage.setItem("次の表示","true");
setTimeout(function()
{window.location.href="b.html";},5000);}};
</script>
<h1>a.html</h1>
<p>5秒後に <b>b.html</b> に進みます。</p>