JavaScript can read the DOM and render a fairly accurate representation of that using canvas. I have been working on a script which converts HTML into a canvas image. Decided today to make an implementation of it into sending feedbacks like you described.

The script allows you to create feedback forms which include a screenshot, created on the client's browser, along with the form. The screenshot is based on the DOM and as such may not be 100% accurate to the real representation as it does not make an actual screenshot, but builds the screenshot based on the information available on the page.

It does not require any rendering from the server, as the whole image is created on the client's browser. The HTML2Canvas script itself is still in a very experimental state, as it does not parse nearly as much of the CSS3 attributes I would want it to, nor does it have any support to load CORS images even if a proxy was available.

Still quite limited browser compatibility (not because more couldn't be supported, just haven't had time to make it more cross browser supported).

For more information, have a look at the examples here:

http://hertzen.com/experiments/jsfeedback/

editThe html2canvas script is now available separately here and some examples here.

edit 2Another confirmation that Google uses a very similar method (in fact, based on the documentation, the only major difference is their async method of traversing/drawing) can be found in this presentation by Elliott Sprehn from the Google Feedback team:http://www.elliottsprehn.com/preso/fluentconf/

Your web app can now take a 'native' screenshot of the client's entire desktop using getUserMedia():

Have a look at this example:

https://www.webrtc-experiment.com/Pluginfree-Screen-Sharing/

The client will have to be using chrome (for now) and will need to enable screen capture support under chrome://flags.

PoC

As Niklas mentioned you can use the html2canvas library to take a screenshot using JS in the browser. I will extend his answer in this point by providing an example of taking a screenshot using this library ("Proof of Concept"):

function report() {  let region = document.querySelector("body"); // whole screen  html2canvas(region, {    onrendered: function(canvas) {      let pngUrl = canvas.toDataURL(); // png in dataURL format      let img = document.querySelector(".screen");      img.src = pngUrl;       // here you can allow user to set bug-region      // and send it with 'pngUrl' to server    },  });}

.container {  margin-top: 10px;  border: solid 1px black;}

<script src="https://cdnjs.cloudflare.com/ajax/libs/html2canvas/0.4.1/html2canvas.min.js"></script><div>Screenshot tester</div><button onclick="report()">Take screenshot</button><div class="container">  <img width="75%" class="screen"></div>