<canvas whidth = "600" height = "400"></canvas>
<script>
var tela = document.querySelector("canvas");
var pincel = tela.getContext("2d");
pincel.fillStyle = "black";
pincel.fillRect (0, 0, 500, 400);
</script>
Solucionado
<canvas whidth = "600" height = "400"></canvas>
<script>
var tela = document.querySelector("canvas");
var pincel = tela.getContext("2d");
pincel.fillStyle = "black";
pincel.fillRect (0, 0, 500, 400);
</script>
<canvas width="600" height="400"></canvas>