<!DOCTYPE html> <style> div { color: red; } div::first-line { color: green } #y { display: inline-block; } </style> <div id="x"><span id="y">green</span></div> <script> x.offsetWidth; var span = document.createElement('span'); span.textContent = 'This should be '; y.insertBefore(span, y.firstChild); </script>