[X-Istence]

HTTP Pipelining has the client sending multiple requests before receiving a response. It turns it into Request, Request, Request, Response, Response, Response.

The problem is that if Request number 1 leads to an error whereby the connection is closed, those latter two requests are discarded entirely. The client would have to retry request number two and three. If the server has already done work in parallel though, it can't send those last two responses because there is no way to specify that the response is for the second or third request.

The only way a server has to signal that it is in a bad state is to return 400 Bad Request and to close the connection because it can't keep parsing the original requests.

There is no support for HTTP pipelining in current browsers.

What you are thinking about is probably HTTP keep alive, where the same TCP/IP channel is used to send a follow-up request once a response to the original request has been received and processed. That is NOT HTTP pipelining.