<!DOCTYPE html>
<style>
details::first-line { color: green; }
* { column-count: 1 }
</style>
<script>
function go() {
  a.appendChild(document.createTextNode("details"))
  b.createTBody()
}
</script>
<body onload=go()>
<table id="b"></table>
<details id="a" open>
<summary hidden></summary>
This is the </details><!-- The entire line "This is ..." should be green. -->