This was pointed out to me by a reader and is an HTTP to MQTT gateway.
It uses an API similar to Paho, The following is taken from the web page here.
The library exposes an Eclipse Paho-like API. Any HTML page can easily become an MQTT client, able to publish and subscribe to/from MQTT topics, irrespective of which MQTT broker you are using. This way, web pages can exchange messages with IoT devices and existing MQTT applications as well as interact with other web pages in real time.
Similarly, any Node.js application will be able to access any MQTT broker and produce/consume messages.