code: func attributeIncludesSelector(key, val string) Selector {
  	return attributeSelector(key,
  		func(s string) bool {
  			for s != "" {
  				i := strings.IndexAny(s, " \t\r\n\f")
  				if i == -1 {
  					return s == val
  				}
  				if s[:i] == val {
  					return true
  				}
  				s = s[i+1:]
  			}
  			return false
  		})
  }
docs: // attributeIncludesSelector returns a Selector that matches elements where
  // the attribute named key is a whitespace-separated list that includes val.