I use the multithread HTML5 worker which will be able to abort an synchronous XMLHttpRequest pointing to an unresponsive URL. This does not block the browser.

sleep.js

function sleep(milliseconds) {
    var xhr = new XMLHttpRequest();
    // Open a non existing url
    xhr.open('GET', location.protocol + '//sleep' + (new Date()).getTime(), false);
    var blob = new Blob([
        "onmessage = function() { setTimeout(function() {postMessage('done');}, "+milliseconds+"); }"]);
    
    var blobURL = window.URL.createObjectURL(blob);
    var worker = new Worker(blobURL);
    worker.onmessage = function() {
        xhr.abort();
        window.URL.revokeObjectURL(blobURL);
    };
    worker.postMessage('sleep');
    xhr.send(null);
};

var start = new Date().getTime();
sleep(2000);
var end = new Date().getTime();
console.log('time: ' + (end - start));