What is the name of the gray blocks that are displayed on the site before the content is loaded (like on YouTube, for example)?

Question

css

I can’t find the name of the term anywhere, when gray blocks are displayed on the page before the content is loaded, in which the content will subsequently appear. A prime example is YouTube. Before downloading the video, we see this:

Actually, what keyword do you need to drive into the search engine to read how it's done? It is the gray blocks themselves.

Answer 1

This thing is called "Skeleton loading" or "Skeleton screens" )

Answer 2

preloadjs is it?