Can someone point where in the source code is the implementation of
the subresources loading and some documentation regarding its
implementation - as a queue or just child-threads or async functions?

The reason is that the current subresource loading seems to lack any
prioritization and it often occurs that some external "1x1 pixel
tracker" or other similarly unimportant page resources block the
rendering of the page completely, and the user is left starting at
"contacting ads.doubleclick.com" with a blank page. This is very
frustrating as the page render as a whole then depends on the slowest
part and cannot be possibly done faster by any optimizations in
hardware or software on the part of the page owner.

Thus, the proposition is this:
1. Render should only wait for the main HTML/CSS to load from the main
page domain (a page in tumblr.com domain should wait for html/css
files from *.tumblr.com, but not from *.doubleclick.com).
2. Other content load except HTML/CSS should be prioritized as
follows, with placeholders shown until the load is complete - possibly
adding one or more extra render passes, but increasing interactivity.

So, basic priorities:

10 = Highest:   HTML/CSS from main domain (sites.tumblr.com/some_site.html)
9: JS/XHR from main domain
8: HTML/CSS/JS from subdomains in the same domain (ads.tumblr.com/ad_serve.js)

7. <Reserved for future use>

6. IMG/media from main domain (sites.tubmlr.com/header.png)
5. IMG/media from subdomains in the same domain (ads.tubmlr.com/banner1.png)

4. <Optional*> HTML/CSS/JS (text) from CDNs
3. <Optional*> IMG/media from CDNs

2. HTML/CSS/JS from other domains
1=Lowest. IMG from other domains (*.doublclick.com/images/track_1x1.gif)

*4 and 3 are optional and would need some kind of a whitelist of
well-known CDN domains.

This prioritization will reduce the latency between the page load
start and a usable render, so even if some external-domain subresource
is nonresponsive, interactivity will not suffer.
Maybe the priorities should be moved to a user-controllable setting,
where more fine-grained rules can be defined. Otherwise, maybe HTML
standard can be extended to provide hints to the browser regarding the
preferred subresource loading order, which should of course be

Thank you for reading.
This might be a big undertaking but the benefit for the user will be
seen instantly.

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