How can I make a block within a block indent so that the first block does not move?

Question

css

https://jsfiddle.net/vsukfzvu/18/
When I write an indent for both elements, although I set only one block. Or not? margin-top: 15px;

Answer 1

Used "collapsing indents"
Options:
1. 2..middle-page { display: flex; }
.middle-page .main-middle-page { float: left; }

Answer 2

Here https://developer.mozilla.org/ru/docs/Web/CSS/CSS_...