<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" >
<defs>
<linearGradient id="gradient" x1="0%" y1="0%" x2="100%" y2="0%">
<stop offset="0%" stop-color="red"/>
<stop offset="100%" stop-color="blue"/>
</linearGradient>
<filter id="shadow" width="200%" height="200%">
<feOffset result="offOut" in="SourceAlpha" dx="75" dy="75"/>
<feGaussianBlur result="blurOut" in="offOut" stdDeviation="10"/>
<feBlend in="SourceGraphic" in2="blurOut" mode="normal"/>
</filter>
</defs>
<path d="M100 100 L400 100 L400 400 L100 400 Z" style="fill:rgba(0,0,0,0); stroke-width:50; stroke:url(#gradient); filter:url(#shadow)"/>
</svg>