How to make 2 divs on one line?

Question

css

I know there are 3 ways:

• Use display: inline-block
  
• Use float: left/right
  
• Use position: absolute
  

In the first block I have the main content, and the second block is the sidebar. You need to set a background for the sidebar. The background should be the entire length of the parent block, regardless of whether the content or the sidebar sets this length.
position: absolute - not suitable, as there is a fixed height.
float: left/right - not suitable, because when setting the height to height: 100%, only the constant height of the parent is inherited.
display: inline-block - here I can't even set it on the same line with constant latitude values ​​( jsfiddle ).

Answer 1

?

Answer 2

jsfiddle.net/65p1c1t2/1
set font-size 0 to the parent block
read about spaces in inline block elements

Answer 3

codepen.io/anon/pen/bdOWze?editors=110
did not go deep, but once this link saved me