Comment by dfabulich

The new element syntax is needed to signify DOM ranges that may cross the boundaries of HTML element trees.

  <p>
    <em>Some <?start name="hl"?>text</em>
    to replace<?end name="hl"?>.
  </p>

Jake Archibald at Mozilla has a good video about this. https://www.youtube.com/shorts/yARSOcqOWvY