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Browsers have a per-host limit regarding number of parallel XHR (about 6 nowadays).
Does this restriction apply to multiplexed HTTP/2 connections?
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As a result, all HTTP/2 connections are persistent, and only one connection per origin is required, which offers numerous performance benefits.
First, because they received requests in large batches instead of smaller, more spread-out batches. And secondly, because with HTTP/2, the requests were all sent together—instead of staggered like they were with HTTP/1.1—so their start times were closer together, which meant they were all likely to time out.
The primary goals for HTTP/2 are to reduce latency by enabling full request and response multiplexing, minimize protocol overhead via efficient compression of HTTP header fields, and add support for request prioritization and server push.
In particular, HTTP/2 is much faster and more efficient than HTTP/1.1. One of the ways in which HTTP/2 is faster is in how it prioritizes content during the loading process.
Browsers impose a per-domain limit of 6-8 connections when using HTTP/1.1, depending on the browser implementation. This allows at most 6-8 concurrent requests per domain.
With HTTP/2, browsers open only 1 connection per domain. However, thanks to the multiplexing feature of the HTTP/2 protocol, the number of concurrent requests per domain is not limited to 6-8, but it is virtually unlimited.
It is virtually unlimited in the sense that browsers and servers may limit the number of concurrent requests via the HTTP/2 configuration parameter called SETTINGS_MAX_CONCURRENT_STREAMS.
Typical limits are around 100 (Firefox's default value for network.http.spdy.default-concurrent - note the spdy name here: it was the protocol ancestor of the HTTP/2 protocol) but could be larger (or, less commonly, smaller), depending on browser implementation and on the server you connect to.
Expect these limits to vary over the years with the evolution and the more widespread usage of HTTP/2 (in the same way it happened with HTTP/1.1: browsers started with 2 connections, and ended up to 6-8 after years of usage, experience and tuning).
I don't think there is any difference between how a browser treats the number of connections and concurrent requests for normal browsing and for the usage of XHR, so the explanations above holds true for XHR as well.
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