How to do what YouTube does with the grey boxes while loading?

Question

I've started to notice that when loading a YouTube page, before content is loaded, everything appears as grey boxes:

Want to try this out and cannot find answers. Only come across articles of people asking for help because the content didn't actually load for them, ex: YouTube Help Forum

Is this YouTube's way of lazy loading? is this mainly done with Javascript?

Answer 1

This technique is also called "skeleton screen".