Audit for minimizing layout shift best practices

[warrengm]

Documentation for minimizing layout shift is up at https://developers.google.com/doubleclick-gpt/guides/minimize-layout-shift. It would be great to audit for some of these best practices on real pages. In particular we could verify that ad slots have some reserved space before being filled. For single-sized slots this size should match the ad size. For multisize it's a bit tricky since there are more tradeoffs. At the very least we should verify that the site is styled to fit the smallest size. (We don't want to audit for the largest size since there are trade-offs between size and blank space or it's possible that the largest size won't fill due to responsive issues)

Technically we want to do this by inspecting computed width/height style on the slot element. More discussion to follow