get html code using javascript with a url

Use jQuery:

$.ajax({ url: 'your-url', success: function(data) { alert(data); } });

This data is your HTML.

Without jQuery (just JS):

function makeHttpObject() {
  try {return new XMLHttpRequest();}
  catch (error) {}
  try {return new ActiveXObject("Msxml2.XMLHTTP");}
  catch (error) {}
  try {return new ActiveXObject("Microsoft.XMLHTTP");}
  catch (error) {}

  throw new Error("Could not create HTTP request object.");
}
var request = makeHttpObject();
request.open("GET", "your_url", true);
request.send(null);
request.onreadystatechange = function() {
  if (request.readyState == 4)
    alert(request.responseText);
};