I think this is great 👍

I also think this is wonderful: especially when combined with #8634, it is a drastic quality of life improvement—thanks a lot!

I also like that without extensions and users doing anything this impementation guarantees that extension assets always come before the user assets, unlike in the previous implementation.

I have the following remarks for consideration:

• This implementation preserves the implicit existing feature that the order of calls to add_js_file and add_css_file with equal priority is the same as the order of asset inclusion. This is guaranteed due to list.sort being stable. The same holds for the user-provided assets (html_js_files / html_css_files). Should this be made into an explicit guarantee and documented?
• The feature seems to be easier to use for extension authors than for end users, since there's no interface for specifying priority in html_js_files and html_css_files. This seems to be quite alright: the advanced users may turn their conf.py into an extension.

Oops. I overlooked @akhmerov 's last comment.

Should this be made into an explicit guarantee and documented?

Indeed. It would be better if documented.

The feature seems to be easier to use for extension authors than for end users, since there's no interface for specifying priority in html_js_files and html_css_files.

I did not think about the user configurations. But it is also allowed to give priority via attribute with this change:

html_css_files = [('custom.css', {'priority': 999}]

I posted #8641 for additional documentation. It would be helpful if you'll review it also (because I'm not good at English...)