Transparent half circle cut out of a div

Question

I would like to make a transparent cut out half circle shape using only CSS3. The only requirement is that all the elements that form the shape must

Answer 1

You can use box-shadows to make the transparent cut out circle :

body {
  background: url(http://i.imgur.com/qi5FGET.jpg) no-repeat;
  background-size: cover;
}
div {
  display: inline-block;
  width: 300px; height: 300px;
  position: relative;
  overflow: hidden;
}
div:before {
  content: '';
  position: absolute;
  bottom: 50%;
  width: 100%; height: 100%;
  border-radius: 100%;
  box-shadow: 0px 300px 0px 300px #000;
}
.transparent {
  opacity: 0.5;
}

This can be responsive with percentage lengths:

body {
  background: url(http://lorempixel.com/output/people-q-c-640-480-1.jpg) no-repeat;
  background-size: cover;
}
div {
  width: 40%; height: 300px;
  position: relative;
  overflow: hidden;
}
div:before {
  content: '';
  position: absolute;
  bottom: 50%;
  width: 100%; height: 100%;
  border-radius: 100%;
  box-shadow: 0px 300px 0px 300px #000;
}
.transparent {
  opacity: 0.5;
}