select_first

# File actionpack/lib/action_controller/vendor/html-scanner/html/selector.rb, line 473
    def select_first(root)
      stack = [root]
      while node = stack.pop
        if node.tag? && subset = match(node, true)
          return subset.first if !subset.empty?
        elsif children = node.children
          stack.concat children.reverse
        end
      end
      nil
    end