In his book "High Performance Websites" Steve Souders wrote (2007) that browsers limit parallel requests to a domain/hostname to two at a time. Is this still valid today?
Almost incorrect today.
Most browsers have upgraded to 6 parallel connections. See Steve Souder's Browserscope > Network tab > Connections per Hostname. Older browsers still restrict to 2 connections - that may or may not be relevant to you depending on the browsers your application supports.
Firefox bumped up its maximum per-server connection count from 8 to 15 for Firefox 3 and later. The maximum is less for servers that support persistent connections:
- Firefox 2: 2
- Firefox 3: 6
- Opera 9.26: 4
- Safari 3.0.4: 4
- IE 7: 2
- IE 8: 6 (except on dial-up)
2 years later the same Steve Souders wrote in "Even Faster Web Sites":
IE8 and FF3 both increase the number of connections per server from two to six.
Yes, there may be some variations, but that limitation is certainly still valid. Some browsers might allow more than two parallel requests to a host, but it's still only a few.
There may also be limitations on the server side. If you are requesting active pages, e.g. ASP/ASP.NET/PHP, they are usually limited to a single request at a time per user.
However, with bandwith increasing this limitation has a smaller impact than it had in 2007. Eventhough you can still only do a few requests in parallel, each request is faster so the limitation is not very noticable.
The HTTP/1.1 spec suggests a cap of 2 simultaneous requests per host (With HTTP/1.0 a figure of 4 was more common).
It would be wise to assume that the client might not be able to have more than 2 simultaneous requests open at once.
来源：https://stackoverflow.com/questions/9753898/is-the-web-browser-performance-rule-only-2-requests-in-parallel-per-hostname-s