<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" width="240px" height="160px" viewBox="0 0 60 40">
  <metadata>
    <link xmlns="http://www.w3.org/1999/xhtml" rel="help" href="https://www.w3.org/TR/SVG2/shapes.html#EllipseElement"/>
    <link xmlns="http://www.w3.org/1999/xhtml" rel="match" href="ellipse-07-ref.svg"/>
    <meta xmlns="http://www.w3.org/1999/xhtml" name="assert" content="ry auto means rx absolute length is used"/>
  </metadata>
  <style>
    ellipse {
      rx: 25%;
      ry: auto;
    }
  </style>
  <ellipse cx="20" cy="20" fill="none" stroke="blue" stroke-width="5"/>
</svg>

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" width="240px" height="160px" viewBox="0 0 60 40">
  <ellipse cx="20" cy="20" rx="15" ry="15" fill="none" stroke="blue" stroke-width="5"/>
</svg>