Web scenarios do not provide Real User Monitoring (RUM), where a scenario runs in a fully-blown Internet browser engine.
Any code supposed to be interpreted or executed in the browser on client side, as it is the case with JavaScript, will have no effect.

A web application with an authentication procedure that requires to get a salted hash POSTed connot be monitored by Zabbix.

E.g.:

var response = challenge + "-" + password;
jxl.setValue("uiResponse", challenge + "-" + hex_md5(response));

In the past I've seen things like these only now and then. Meanwhile, I'm encountering more and more web services that cannot be monitored with Zabbix.

So, how about adding embedded script support on step level?

PS: It seems reasonable to use JavaScript but I think other interpreters like for instance LUA could help here too