Image requests from CSS background-image

I’d like to know what proportion of images requests come from CSS rather than img elements.

I found this from a few years ago, but it would be great to have something with wider browser coverage: Who initiates image downloads?

Here’s a quick and dirty query that only takes into account requests from files with “.css” in the name:

select url, mimetype, req_referer, respsize
from `httparchive.summary_requests.2019_06_01_mobile` 
where mimetype like "%image%" and req_referer like "%.css%"
order by respsize desc

I get 18M images are requested by discrete CSS files (it would certainly be higher if we included inline CSS).

removing he CSS requirement (to find total images in the sample) fails (too big a response), but this query does work:

select count(url) cnt
from `httparchive.summary_requests.2019_06_01_mobile` 
where mimetype like "%image%" 

returns 189M images in the dataset. So it appears that about 9.5% of images are requested from a CSS file.