Change 339095 had a related patch set uploaded (by Krinkle):
[WIP] Initial POC for detecting unused images in style rules

https://gerrit.wikimedia.org/r/339095

This is a really small nitpick and we can perfectly release it without fixing it in my book, but in Chrome the URL slips down a couple of pixels:

When I tested in Firefox it seems the same images is picked up twice:

But when I test on articles with a lot content, it seems like inspector is broken for Firefox, let me investigate that.

The Firefox issue is CSP problem right? I'll create task for catch that and log it, but something has changes since "it worked before"™.

In T121144#3083892, @Peter wrote:

When I tested in Firefox it seems the same images is picked up twice:

Served stylesheet:

.postedit-icon-checkmark {
	background-image: url(data:image/png;base64,iVBORw0KGg..);
	background-image: url(/w/resources/src/mediawiki.action/images/green-checkmark.png?d94f1)!ie;
}

In Chrome, only the effective rules are iterated over. Later declarations override previous ones, except if they're invalid. Which is a common way of setting background fallbacks (e.g. first set a solid background color, then try again with rgba).

In Firefox, I thought it iterated over both. However, adding some instrumentation showed that it was due to something else entirely:

Firefox provides access both through the camelCase and the dashed-name of the parsed CSS rules. As accessor this makes a lot of sense, but not for iteration :facepalm:.

Fixed in the latest patch version of https://gerrit.wikimedia.org/r/339095.

Change 339095 merged by jenkins-bot:
[mediawiki/extensions/PerformanceInspector] Detect unused image embeds in stylesheets

https://gerrit.wikimedia.org/r/339095