Phantomjs handles everything you mention (status codes on large numbers of resources, ajax, deferred loading monitoring and HAR output) with the possible except

Phantomjs handles everything you mention (status codes on large numbers of resources, ajax, deferred loading monitoring and HAR output) with the possible exception of websockets - I have not tried and very little documentation today but it should work. The big limitation is this is WebKit-only right now. For example: here's the wiki on network monitoring including HAR: https://github.com/ariya/phantomjs/wiki/Network-Monitoring https://github.com/ariya/phantomjs/wiki/Network-Monitoring The API seems pretty clean to me but I guess that is a matter of opinion.