<!-- filters.svg -->
  <svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
    <!-- define the filter #blur -->
    <filter id="blur">
      <feGaussianBlur stdDeviation="2"/>
    </filter>

    <!-- use the filter #blur -->
    <rect x="10" y="10" width="80" height="80" filter="url(#blur)"/>
  </svg>