Indicate observed and expected sizes for "Properly size images"

[danielbachhuber]

Feature request summary

"Properly size images" is pretty good at identifying incorrectly sized images.

However, it (and the linked documentation) leaves the reader in the dark on what their image sizes should actually be.

As such, it would be helpful if "Properly size images" indicated the observed size and expected size(s) for a given image.

Related #11593 (comment)

What is the motivation or use case for changing this?

I'd like for my WordPress site to use correct sizes for all of its image positions. However, it's challenging to compute what the srcset value should actually be.

This bookmarklet is pretty helpful for determining what the sizes attribute should be: https://ausi.github.io/respimagelint/

Related ausi/respimagelint#62

How is this beneficial to Lighthouse?

If it was easier to identify the expected image sizes, more sites would pass this audit step.