code: function SpanSerializer(props) {
      const {mark, children} = props.node
      const isPlain = typeof mark === 'string'
      const markType = isPlain ? mark : mark._type
      const serializer = props.serializers.marks[markType]
      if (!serializer) {
        // @todo Revert back to throwing errors?
        // eslint-disable-next-line no-console
        console.warn(
          `Unknown mark type "${markType}", please specify a serializer for it in the \`serializers.marks\` prop`
        )
        return h(props.serializers.markFallback, null, children)
      }
  
      return h(serializer, props.node, children)
    }
docs: Low-level span serializer