Scale A Rectangle In Html And Css

I want to make a rectangle (a div) with a specific scale. The height-width scale has to be 30:50. The rectangle fills 40% of the width-screen. If I resize my screen, how can I chan

Solution 1:

Try scaling it using vw (viewport width):

#bg {
  background-color: blue;
  width: 40vw;
  height: 66.67vw;
}

https://jsfiddle.net/gmsitter/9tpbq30x/1/