code: func (h *highlightHTMLRenderer) ListItem(out *bytes.Buffer, text []byte, flags int) {
  	switch {
  	case bytes.HasPrefix(text, []byte("[ ] ")):
  		text = append([]byte(fmt.Sprintf(`<input class="markdown-checkbox" type="checkbox" disabled="disabled" data-checkbox-index="%d"></input>`, h.checkboxIndex)), text[4:]...)
  		h.checkboxIndex++
  	case bytes.HasPrefix(text, []byte("[x] ")) || bytes.HasPrefix(text, []byte("[X] ")):
  		text = append([]byte(fmt.Sprintf(`<input class="markdown-checkbox" type="checkbox" disabled="disabled" checked=""  data-checkbox-index="%d"></input>`, h.checkboxIndex)), text[4:]...)
  		h.checkboxIndex++
  	}
  	h.Renderer.ListItem(out, text, flags)
  }
docs: // ListItem overrides the default ListItem render and adds support for GH-style
  // checkboxes on service side. For the contents of the list item beyond the
  // checkbox prefix, the default Html.ListItem is called.
  // This adds a data-checkbox-index attribute that contains the ordinal of the
  // checkbox in the rendered Markdown. This can be used by the ui to find the
  // reference to the original Markdown code from a rendered HTML.