The factors than make an element Contentful or not-Contentful should be defined

[andydavies]

The spec states that

This document defines an API that enables monitoring the largest paint an element triggered on screen.

But Chromium doesn't follow this definition and for example excludes full viewport images (https://chromium.googlesource.com/chromium/src/+/refs/heads/main/docs/speed/metrics_changelog/2020_11_lcp.md)

So both consumers and implementers of the spec have clarity and consistency on what is actually, then what is considered contentful and any exclusions should be defined

[clelland]

This could be an umbrella issue for #26, #28, #70, #72, #79, and #107.

There are a number of reasons that a paint would not be considered 'contentful' for LCP purposes, and we should define those in the spec. I'll take a pass at a PR or 6 which makes them more explicit.