First, I want to make clear that I am not talking about this question How many concurrent AJAX (XmlHttpRequest) requests are allowed in popular browsers? , which is about simultaneous requests.
Instead, I want know if there is a limit on the number of pending ajax requests, before the browser potentially starts cancelling them or throwing errors.
Running some simple tests, I've seen that when the limit of about 6 simultaneous requests is hit, the browser will start queuing requests in a graceful manner. Once a 'slot' becomes available, it is used and a new request is sent to the server.
I have also seen that Chrome, Firefox and even IE will gracefully handle a queue of about 100 requests.