<!DOCTYPE HTML>
<html>
<head> </head>
<body>
<input id="search-tag" placeholder="Какой тэг искать будем?" type="text">
<input onclick="searchTag()" type="button" value="ОК">
<p>Найдено: <span id="result">?</span></p>
<script type="text/javascript">
function searchTag() {
searchTag.result.innerHTML = document.getElementsByTagName(searchTag.input.value || '*').length;
};
searchTag.input = document.getElementById('search-tag');
searchTag.result = document.getElementById('result');
</script>
</body>
</html>