Ask HN: How do you test in CI/CD of large content sites?

For very large sites with thousands of dynamic pages, what are some ways to test the consistency of UI elements, layout, SEO conventions, etc... between builds and environments?

I recently stumbled on a very large WP project for which I've had to build a custom "diffing" tool between different environments for every page / article. I am curious to learn about how other large site owners solve this issue.