code: func (s Selector) MatchFirst(n *html.Node) *html.Node {
  	if s.Match(n) {
  		return n
  	}
  
  	for c := n.FirstChild; c != nil; c = c.NextSibling {
  		m := s.MatchFirst(c)
  		if m != nil {
  			return m
  		}
  	}
  	return nil
  }
docs: // MatchFirst returns the first node that matches s, from n and its children.