CSS

css calc

.content {
  width: calc(100% - 300px);
}

css calc function

.banner {
  position: absolute;
  left: 40px;
  width: calc(100% - 70px);
  border: solid black 1px;
  box-shadow: 1px 2px;
  background-color: yellow;
  padding: 6px;
  text-align: center;
  box-sizing: border-box;
}

calc() css

.banniere {
  position: absolute;
  left: 40px;
  width: calc(40px - 80px);
  border: solid black 1px;
  box-shadow: 1px 2px;
  background-color: yellow;
  padding: 6px;
  text-align: center;
  box-sizing: border-box;
}