How to implement page structure preview?

Question

User interface

Greetings!
Perhaps someone knows how to implement a preview of the page structure before it is fully loaded?
To make it clear what I'm talking about, I'm attaching a screenshot:

Answer 1

Nail the blocks that will definitely be on the page to the places allotted to them, give them some reasonable minimum sizes, throw a square and a few stripes on the background - voilà.