#div1 {
  position: absolute;
  left: 50px;
  width: calc(100% - 100px); /* 50px de cada lado */
  border: 1px solid black;
  background-color: yellow;
  padding: 5px;
}

#div12 {
  background-color: yellow;
  height: 100px;
  width: max(50%, 300px);
}


#div13 {
  background-color: yellow;
  height: 100px;
  width: min(50%, 300px);
}