This is one of those optimizations where I'm ambivalent about it's usefulness.
Sure, Google might get a millisecond or two less in their apps, but who in their right mind has the engineering time to implement this very, very small optimization when there's many other things to be done that probably have a higher priority?
I could imagine it is a very good optimization for sites that take a long time to return the HTML of the page.
Take a site like reddit, where the HTTP request can take a second or 2 to return when there is a really high server load.
Reddit could return HTTP 103 hints to the clients to start downloading useful assets ahead of time so when the HTML payload finally arrives everything is ready to start execution. And if this does become a standard, it can get built into web servers and will be as simple as adding a few lines to a config file.
Obviously the "correct" solution is to reduce the time it takes to return the HTML from the server, but if it can improve load times for a nonzero amount of people for not much work, it's a win in my book (even if it's not ideal).
You are right, most people don't have the engineering time to implement this. Though there is definitely a market for this kind of optimizations in a SaaS solution (I know that because it has something to do with my job)...
Now, implementation of this by browsers is going to take a long time, and then we will see incompatibilities trickle down all the way, with Safari taking a few years after everybody else has it and IE/Edge also taking forever.... I should rather work in a farm and harvest tomatoes...
"millisecond or two" won't cut it as the only usable case would happen when there is non-trivial delay between 103 and 200 statuses.
Also I see this as more harmful than good as it'd require yet another packet sent and ack'd.
I also fail to see the usefulness. A server can already today send only the html head, flush the stream then pause before sending the body?
Most modern SPAs also have a static index.html which shouldn't take long to serve anyway.
This is going to be abused by cargo cult brogrammers who heard that "103 will make your site faster!!!1" and put every single resource as a 103 making the time to 200 even longer than the time you actually potentily gain.
With the article yesterday about the Trinet[1] I can see this optimization being used in the majority of HTTP traffic, even if it won't be used by a majority of companies/websites.
My reading of it is that it also increases the utility of centralized resources like CDNs and Google fonts relative to hosting your own resources, which HTTP/2 increases the utility of. In other words, this tends to promote the centralization of the web and is a gift to big tech companies.
Sure, Google might get a millisecond or two less in their apps, but who in their right mind has the engineering time to implement this very, very small optimization when there's many other things to be done that probably have a higher priority?