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