#parent {
  background-color: red;
  height: 5000px;
}

#child1 {
  background-color: yellow;
}

#child2 {
  background-color: lightgreen;
  position: fixed;
  top: 0; 
  left:0;
}

#child3 {
  background-color: lightblue;
}

#parent {
  max-width: 300px;
}
#child1,#child2, #child3 {
  max-width: 200px;
}