[contentEditable] {
  white-space: pre-wrap;
  word-break: break-all;
}

document.querySelector('[contentEditable]').addEventListener('keydown', function(event) {
  if (event.key === 'Enter') {
    document.execCommand('insertHTML', false, '<br>');
    event.preventDefault();
  }
});

document.execCommand('defaultParagraphSeparator', false, 'br');