<!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. -->