var html_text = document.getElementById("textarea1").innerHTML;
/* the textarea contains tags like <b>test</b> */
var blob = new Blob([html_text], {
 type: "text/html; charset=utf-8"
});
var url = URL.createObjectURL(blob);
var a = document.createElement('a');
a.download = "new.html";
a.href = url;
a.textContent = "Download";
document.getElementById("aDiv").appendChild(a);