Auto-exclude Trust Index Google Widget CSS from LazyLoad for CSS Background Images

[webtrainingwheels]

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Describe the bug
When using the Trust Index Google Widget plugin and LazyLoad for CSS background images, it creates a performance problem, especially on mobile.

The CSS file from that plugin contains A LOT of images. The resulting inline JS that we create is therefore very large.
This results in:

• A high blocking time
• Significant main thread work
• High JS execution time

This problem could occur in any scenario where there are an excessive number of images in CSS, but it's very consistently happening with this specific plugin. Disabling the option improves performance in those cases.

To Reproduce
Steps to reproduce the behavior:

1. Add this CSS file to your site: https://gist.github.com/webtrainingwheels/022e66c2f019142c99403a54d7d0b97d
2. Enable LazyLoad for CSS Background Images
3. Check the source code for the big inline script: https://jmp.sh/Ledw0Iu1
4. Run a PageSpeed test and note the high blocking time, main thread work and JavaScript execution time: https://jumpshare.com/v/opd03dvcDIWVJFApH9kH, https://jumpshare.com/v/3EPMC794PYdLMmh2BNVx

Additional context
Slack convos:
https://wp-media.slack.com/archives/C08N8J6VC/p1724876333198109
https://wp-media.slack.com/archives/C8L4EE8N9/p1725582448549079

Related tickets:
https://secure.helpscout.net/conversation/2699519664/510946/
https://secure.helpscout.net/conversation/2689724995/509272/
https://secure.helpscout.net/conversation/2692407860/509723/
https://secure.helpscout.net/conversation/2681783608/507952?folderId=8127822
https://secure.helpscout.net/conversation/2697455510/510593?folderId=7406897
https://secure.helpscout.net/conversation/2688555094/509064?folderId=8127840

Acceptance Criteria (for WP Media team use only)
Clear instructions for developers, to be added before the grooming

[camilamadronero]

Another case https://secure.helpscout.net/conversation/2706156322/512083/

[patrick-7-7-7]

My workaround is this in an mu-plugin (well, use it in some plugin or child theme):

<?php
add_filter("rocket_lazyload_css_ignored_urls", "excludeAllOfTrustindexFromLazyloadedCssImages", 10, 1 );

function excludeAllOfTrustindexFromLazyloadedCssImages($orig){
        $added = [
                "trustindex"
        ];
        $orig += $added;

        return $orig;
}

Actually, I use this array, but I have not tested whether there is a performance benefit. A few sites have 50 such images.
$added = [
'plugins/fusion-builder/assets/images/iLightbox',
'trustindex',
'wp-media-folder',
'plugins/all-in-one-event-calendar/public/',
]

Some sanity checking in the WP Rocket code would be nice. The most boring solution is to just set a limit of 50.

[edited to fix formatting]

[camilamadronero]

Another case https://secure.helpscout.net/conversation/2712580830/513207/
Setting these as LazyLoad for CSS background images exclusions worked:

trustindex-google-widget.css
cdn.trustindex.io

[camilamadronero]

New case: https://secure.helpscout.net/conversation/2716762280/514036?folderId=2675957

[worldwildweb]

related: https://secure.helpscout.net/conversation/2726494427/516109?folderId=5547167

[camilamadronero]

Another case https://secure.helpscout.net/conversation/2736295136/518202/

[camilamadronero]

Another one: https://secure.helpscout.net/conversation/2737179931/518366/

[camilamadronero]

Rel: https://secure.helpscout.net/conversation/2734821096/517921/
The site's loading time was hugely impacted by this
We had to disable LL BG totally to fix that slowness problem

[piotrbak]

AC:
Exclude the following patterns from the LazyLoad Background CSS:

trustindex-google-widget.css
cdn.trustindex.io