Defer offscreen images includes a css file instead of only images

### FAQ

- [X] Yes, my issue is not about [variability](https://github.com/GoogleChrome/lighthouse/blob/master/docs/variability.md) or [throttling](https://github.com/GoogleChrome/lighthouse/blob/master/docs/throttling.md).
- [X] Yes, my issue is not about a specific accessibility audit (file with [axe-core](https://github.com/dequelabs/axe-core) instead).

### URL

https://developers.google.com/speed/pagespeed/insights/?url=https%3A%2F%2Fwww.otzerling.com%2F

### What happened?


### What did you expect?

To not consider a css file as an image.

### What have you tried?

_No response_

### How were you running Lighthouse?

PageSpeed Insights

### Lighthouse Version

8.4.0