html,
body {
  width: 100%;
  height: 100%;
}
#grid {
  width: 300px;
  height: 300px;
  background-color: black;
}

path {
  stroke: red;
  stroke-width: 5px;
}
#demo {
  width: 800px;
  height: 500px;
  background: grey;
}

.box {
  width: 50px;
  height: 50px;
  background: white;
  margin: 0;
  padding: 0;
  display: inline-block;
}