code: func (p *parser) parseIDSelector() (Selector, error) {
  	if p.i >= len(p.s) {
  		return nil, fmt.Errorf("expected id selector (#id), found EOF instead")
  	}
  	if p.s[p.i] != '#' {
  		return nil, fmt.Errorf("expected id selector (#id), found '%c' instead", p.s[p.i])
  	}
  
  	p.i++
  	id, err := p.parseName()
  	if err != nil {
  		return nil, err
  	}
  
  	return attributeEqualsSelector("id", id), nil
  }
docs: // parseIDSelector parses a selector that matches by id attribute.