docs: function handler(req, res, next) {
      next = next || send
  
      if (pending) return pending.once('ready', function(err) {
        if (err) return next(err)
        return handler(req, res, next)
      })
  
      if (prevError) return next(prevError)
  
      res.setHeader('content-type', 'text/css')
      return next(null, buffer)
  
      function send(err, body) {
        if (err) return res.emit(err)
        res.end(body)
      }
    }