You need to set a viewBox rather than a fixed width and height.
<svg xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 1000 20" preserveAspectRatio="none" width="100%" height="100%">
<polygon points="0,0 1000,0 500,20"
style="fill:#cc3333;"/>
</svg>