static L<IntRange> charRangesOfHTMLComments(S html) {
  ret charRangesOfHTMLComments(htmlTok(html));
}

static L<IntRange> charRangesOfHTMLComments(LS tok) {
  new L<IntRange> l;
  int pos = 0;
  for idx over tok: {
    S t = tok.get(idx);
    if (even(idx)) {
      int i = 0;
      while ((i = indexOf(t, "<!--", i)) >= 0) {
        int j = smartIndexOf(t, "-->", i+4);
        l.add(intRange(pos+i, pos+j+3));
        i = j+3;
      }
    }
    pos += l(t);
  }
  ret l;
}

Began life as a copy of #1028644