How can we make images flexible (core RWD concept 2)?

If the width property is set to a percentage and the height property is set to "auto", the image will be responsive and scale up and down:

img {
  width: 100%;
  height: auto;
}

If the max-width property is set to 100%, the image will scale down if it has to, but never scale up to be larger than its original size:

img {
  max-width: 100%;
  height: auto;
}

Source: https://www.w3schools.com/css/css_rwd_images.asp