html

<svg viewBox="0 0 200 200" xmlns="http://www.w3.org/2000/svg">
  <defs>
    <foreignObject id="object1" x="50" y="75" width="100" height="50">
      <div xmlns="http://www.w3.org/1999/xhtml">
        <p>This is a foreign object.</p>
      </div>
    </foreignObject>
    <foreignObject id="object2" x="25%" y="50%" width="10%" height="10%">
      <div xmlns="http://www.w3.org/1999/xhtml">
        <p>This is another foreign object.</p>
      </div>
    </foreignObject>
  </defs>
  <rect x="0" y="0" width="200" height="100" fill="lightblue" />
  <rect x="0" y="100" width="200" height="100" fill="lightgreen" />
</svg>

js

const foreignObjects = document.querySelectorAll("foreignObject");
const widthObject1 = foreignObjects[0].width;
const widthObject2 = foreignObjects[1].width;

console.dir(widthObject1.baseVal.value); // output: 100
console.dir(widthObject2.baseVal.value); // output: 20 (10% of 200)