<script>
function MKCode(whatsToDo) {
var htmlCode = document.getElementById("code-html").value
if (whatsToDo=="New") {
document.getElementById("MakedCode").innerHTML = htmlCode
}
*!*
else {
document.getElementById("MakedCode").innerHTML += htmlCode
*/!*
}
}
</script>
<textarea id="code-html"><!--Ваш код--></textarea>
<br/>
<input type="button" onclick="MKCode('New')" value="Заместить код"/>
<br/>
<input type="button" onclick="MKCode('+')" value="Добавить код"/>
<br/>
<div style="Background: #cde; text-align: center" id="MakedCode"></div>