<style>
    canvas { border: solid green;
             zoom: 2; }
</style>
<p>
    These should be four green hollow boxes with dimensions 600x300, 100x300, 600x100, 100x100.
</p>
<!-- 300x150 -->
<canvas id="canvas"></canvas>
<!-- 50x150 -->
<canvas id="canvas" width="50"></canvas>
<!-- 300x50 -->
<canvas id="canvas" height="50"></canvas>
<!-- 50x50 -->
<canvas id="canvas" width="50" height="50"></canvas>