> Is it really better than just having a placeholder square with maybe the most prominent color? Or just a good old progressive image?

The Hugo static site generator provides a `.Colors` method[0] for doing this. It produces what one 2018 article[1] referred to as a GIP (Gradient Image Placeholder).

[0]: https://gohugo.io/methods/resource/colors/#article

[1]: https://calendar.perfplanet.com/2018/gradient-image-placehol...