Warn when CWV antipatterns are detected

[rviscomi]

This post covers a few of the most effective CWV optimizations. Add the ability for the extension to console.warn when the page isn't following best practices.

A good proof of concept to start might be lazy-loaded LCP images.

[tunetheweb]

I'm concerned about all the nuances we might need here.

For native lazy-loading images that's probably fairly obvious and OK. For JS lazy-loading that takes more logic. And for delayed loading due to injecting the LCP element and/or having it in CSS that's more knowledge.

Then for fetchpriority we don't want to create more noise by pushing that on people if it won't even make a difference (e.g. it arrives, even before the images is needed).

There's often a lot of nuance in Lighthouse audits to handle these things and don't think we should be aiming to replicate that here.

[rviscomi]

Yeah, let's start with native lazy loading.