html, body {
   height: 100%;
   width: 100%;
   margin: 0;
}

h1 {
   height: 40px;
   background-color: #ee6;
   margin-block-start: 0;
   margin-block-end: 0;
   text-align: center;
   box-sizing: border-box;
}

#map {
   height: calc(100% - 40px);
   width: 100%;
   box-sizing: border-box;
}