<html>
<head>
<style>
span { display: inline-block; font-family: monospace; }
</style>
<script>
function test()
{
   var summary = document.getElementById("s2");
   var text = summary.previousSibling;
   text.data = " ";
}

function runTest()
{
    if (window.testRunner)
        document.getElementById("run").click();
}
</script>
<body onload="runTest()">
<details open>
  <span id="s1">Details1</span>
  <summary>Summary</summary>
  Text<span id="s2">Details2</span>
</details>
<input id="run" type="button" value="click" onclick="test()">
</body>
</html>