Question: How can I make requests to other server(s) (i.e. get a page from any desired server) with a JavaScript within the user's browser? There are limitations in place to prevent this for methods like XMLHttpRequest, are there ways to bypass them or other methods?

That is a general question, specifically I want to check a series of random websites and see if they contain a certain element, so I need the HTML content of a website without downloading any additional files; all that in a JavaScript file, *without any forwarding or proxy mechanism on a server*.

(Note: one way is using Greasemonkey and its GM\_xmlhttpRequest.)

Answer: 
You can also use a iframe to emulate an ajax request. This saves you the mess of having to code a Backend solution for a Frontend problem. Here is an example:

```
function setUploadEvent(typeComponet){
       var eventType = "";
       var iframe = document.getElementById("iframeId");
       try{
           /* for Mozilla / Opera9 */
           if (/(?!.*?compatible|.*?webkit)^mozilla|opera/i.test(navigator.userAgent)) {
                eventType = "onload";
           }else{
           /* IE  */
                eventType = "onreadystatechange";
           }

           iframe[eventType] = function(){
                var doc = iframe.contentDocument || iframe.contentWindow.document;
                var response = doc.body.innerHTML; /* or what ever content you are looking for */
             }
           }
       catch(e){
           alert("Error loading content")}
       } 

```

That should do the trick. Please note that the Browser detection line is not the cleanest, you should absolutely use the ones provided in all the most common JS frameworks (Prototype, JQuery, etc)
You will need to write a proxy on the server to do this. And all requests will be to your server and then your server will load html and send it back to client. And there are no good way to implement this via javascript only.  

jQuery contains functionality to load JSON data or external scripts using XmlHttpRequest but this functionality can not be used for html pages. Also you may check [this thread](http://groups.google.com/group/jquery-en/browse_thread/thread/130a8df99afdf9c4/82d6431c96cd14f5) of jQuery mailing list.