How to resize an image to fit in the browser window?

Question

This seems trivial but after all the research and coding I can\'t get it to work. Conditions are:

1. The browser window size is unknown. So please don\'t propose

Answer 1

Make it simple. Thanks

.bg {
  background-image: url('https://images.unsplash.com/photo-1476820865390-c52aeebb9891?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&w=1000&q=80');
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center;
  height: 100vh;
  width: 100vw;
}