class Sanitize::CSS

def at_rule!(rule)
  name = rule[:name].downcase
  if @at_rules_with_styles.include?(name)
    styles = Crass::Parser.parse_rules(rule[:block],
      preserve_comments: @config[:allow_comments],
      preserve_hacks: @config[:allow_hacks])
    rule[:block] = tree!(styles)
  elsif @at_rules_with_properties.include?(name)
    props = Crass::Parser.parse_properties(rule[:block],
      preserve_comments: @config[:allow_comments],
      preserve_hacks: @config[:allow_hacks])
    rule[:block] = tree!(props)
  elsif @at_rules.include?(name)
    return nil if name == "import" && !import_url_allowed?(rule)
    return nil if rule.has_key?(:block)
  else
    return nil
  end
  rule
end