CODE EXAMPLE FOR CSS

svg width

<!-- If you actually want it to stretch horizontally, 
disable aspect ratio preservation with preserveAspectRatio=none -->

<svg width="100%" height="50px" viewBox="0 0 20 10" preserveAspectRatio="none">
    <polygon fill=red stroke-width=0 
             points="0,10 20,10 10,0" />
</svg>