how to maintain aspect ratio using CSS?

You can use vw unit to specify width & height, it will make your div aspect ratio using CSS according to the viewport width(vh is used to keep aspect ratio according to viewport height)

1vw : 1%  of the width of the viewport, check here

To keep 1:1 aspect ratio viewport css would be

div{
  width: 25vw;
  height:25vw;
  background:yellow;
}

HTML :  <div></div>

For other aspect ratios, you can use the following table to calculate the value for height according to the width of the element:

aspect ratio  |  multiply width by
-----------------------------------
     1:1                |         1
     1:3                |         3
     4:3                |        0.75
    16:9              |       0.5625

Check this fiddle example.I hope this works for aspect ratio using css, thanks