How to make a div fill a remaining horizontal space?

Question

I have 2 divs: one in the left side and one in the right side of my page. The one in the left side has fixed width and I want the one of the right side to f

Answer 1

You can use the Grid CSS properties, is the most clear, clean and intuitive way structure your boxes.

#container{
    display: grid;
    grid-template-columns: 100px auto;
    color:white;
}
#fixed{
  background: red;
  grid-column: 1;
}
#remaining{
  background: green;
  grid-column: 2;
}

  
Fixed
  
Remaining