body {
  width: 1000px;
  margin: 0 auto;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}
header {
  height: 150px;
  background-color: lightgreen;
  font-size: clamp(4px, 2vw, 20px);
}
div {
  display: flex;
  flex: 1;
}
main {
  flex: 1;
  
}
#footer {
  width: 1000px;
  font-size: 12px;
  color: #aaa;
  padding: 5px;
  border-top: 2px dotted #ccc;
  box-sizing: border-box;
}
@media screen and (max-width: 800px) {
  body {
    width: 100%;
  }

}