what will block a request of the browser?

Question

As i konw, if connections between browser and the server is used up,the request will be blocked .
i read http://www.stevesouders.com/blog/2010/02/07/browser-script-loading-roundup/
js will block the page render ,it won't be parallel downloaded with some other resource like image,iframe(different between browsers)
my question is:

1. is there any other condition will make it?
2. is it means js will block the request for iframe?

expect your help! thanks

Answer 1

This has nothing do with JS. It's a browser safety mechanism. Consider a page that has 500 images in it. To fetch that page would (theoretically) require 501 connections - one for the html, and 500 for the images. If browsers didn't limit the number of requests, 1000 users hitting the page at the same time will perform 501,000 http requests at the same moment and kill the server.

So yes, if the total number of external resources on a page (html, images, scripts, media files, etc...) exceeds the browser's TCP connection limit, some of those resources will have to wait in a queue before they're fetched.