Eclipse Paho MQTT Javascript Client test in a TabrisJS mobile application. Paho is smaller and ready to use in browser than mqtt.js browserified version.
Some of the online test brokers support websockets.
iot.eclipse.org :80 https://www.eclipse.org/paho/clients/js/utility/
broker.mqttdashboard.com :8000 http://www.hivemq.com/demos/websocket-client/
q.emqtt.com :8083 https://github.com/emqtt/emq-dashboard