Glad others are starting to articulate this issue. HTTP/3 is derived from HTTP/2. Google's main argument for HTTP/2's existence, its selling point to users, is head-of-line blocking in HTTP/1.1 pipelining. They also complain about the size of repeated HTTP headers.

But no modern browsers actually use HTTP/1.1 pipelining. Interestingly, HTTP/1.1 pipelining works great for non-browser use. Most web servers enable it by default. After all, it works. For example requesting a series of pages from multi-page website, all under a single TCP connection. I have been using HTTP/1.1 pipeplining this way for decades. It is fast and reliable and enables the web to be used as a non-interactive, information retrieval source. It is also 100% ad-free. The user only gets what she requests, nothing more.

As for HTTP headers, privacy-conscious or minimalist users might not send many headers, only the minimum to retrieve the page. That's usually up to three extra lines of text per page for the request headers. (I rarely ever have to send a User-Agent header for HTTP/1.1 pipelining.)

GET /index.html HTTP/1.1
Host: example.com
Connection: keep-alive

Obviously, the web advertising/tracking industry, including companies like Google that serve this sector, use headers for their own purposes. Online advertising services. That's when presumably they could get big. However, as a user, I have no pressing need for the ability to send/receive larger headers.

Websites (IPs represented by domain names) to which users intentionally connect, i.e., the recognisable names that they type and click on, generally don't serve ads. The ads come from other domains, often other servers. Users generally do not intentionally try to connect to ad or tracking servers. HTTP/[01].x's automatic loading of resources, Javascript and other techniques may be used to make those requests, conveniently under the radar and outside the user's awareness.

Still, under HTTP/1.1, ads, nor Javascript files that trigger requests for ads, generally cannnot be delivered without the user's computer making a request first. Users can and do manage to exercise some control over their computers and they can prevent these non-interactive requests from being sent, from inside and outside the browser.

With HTTP/2 and HTTP/3, the necessity of a user-generated request disappears. As soon as the user "connects" (UDP) to the website's server, the server could for example send a Javascript file to the user's browser which can in turn trigger requests to other domains for ads or the purpose of tracking, all without any preceding request for the ad/tracking-related Javascript file. This is another feature of HTTP/[23] called "server push", but interestingly it is not the feature being used to sell HTTP/[23] to users (i.e., pipelining).

So, how does a user stop unwanted ads being "pushed" upon her in the stream (irrespective of the application, e.g., browser)? I generally don't use a "modern" browser, nor Javascript nor graphics. I like my pipelining outside the browser and free of advertising-related cruft.

It's worth considering that the motivation for speeding up websites via HTTP/[23] is solely for the purpose of speeding the delivery of more ads, more "stealthily", to users. This is a classic case of someone trying to sell you on a "solution" to a problem they themselves have created (or to which they are contributing).

Like an ISP trying to upsell customers to faster internet in order that websites bogged down with ads will "load" faster. When the ISP itself injects ads into pages of websites that are weighed down by ads.