Remove erroneous check for resource initiator type when considering whether to submit LCP background image

[westonruter]

Summary

When setting up a demo site with Optimization Detective, I was confused when I wasn't getting the lcpElementExternalBackgroundImage data submitted with the URL Metric. In looking at the console, I saw:

This was strange because the LCP element was definitely using a background-image coming from a stylesheet:

Nevertheless, the initiatorType of the resource was coming through as an img and not as css. Then I looked further down the page and I found the same image was being used in an IMG tag:

This actually reveals an interesting performance fact: even though the image is being used as a background image for the LCP element, there is an image way further down the page which is actually initiating the loading of this image before the stylesheet is initiating the load. Even more surprising, this IMG has loading=lazy. This highlights the need for adding a preload link for this image URL so that it is properly prioritized!

Granted, it may be unlikely for the exact same image used as the LCP element's background to also appear further down the page in non-example content. In any case, checking the initiatorType for the resource was redundant because it's already aborting if the LCP element is an IMG tag:

performance/plugins/image-prioritizer/detect.js

Lines 90 to 98 in 2033d44

	// Look only for LCP entries that have a URL and a corresponding element which is not an IMG or VIDEO.
	if (
	! entry.url ||
	! ( entry.element instanceof HTMLElement ) ||
	entry.element instanceof HTMLImageElement ||
	entry.element instanceof HTMLVideoElement
	) {
	continue;
	}

Therefore, there is no need for the additional check.

[github-actions]

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Co-authored-by: westonruter <[email protected]>
Co-authored-by: felixarntz <[email protected]>

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[felixarntz]

@westonruter Great catch!