Background color of text in SVG

<svg width="100%" height="100%">
  <defs>
    <filter x="0" y="0" width="1" height="1" id="solid">
      <feFlood flood-color="yellow" result="bg" />
      <feMerge>
        <feMergeNode in="bg"/>
        <feMergeNode in="SourceGraphic"/>
      </feMerge>
    </filter>
  </defs>
  <text filter="url(#solid)" x="20" y="50" font-size="50">solid background</text>
</svg>