This is very similar to ZBXNEXT-2172.

One major issue with more complex web scenarios is the fact that one is almost flying "blind".

You always only know at which step a web scenario failed and which criteria wasn't met.
But there is no information that might help (a frontend user) getting the root cause of an issue, which is often indicated by the web server's/application's response.
The upcoming feature of ZBXNEXT-101 may help but could only be considered as a work-around for privileged users and is of no added value to frontend users - what this issue is aimed to achieve

Well, capturing and storing the response of every response would be great, but is actually no option in terms of performance and efficiency. Even when having in mind that ZBXNEXT-844 is right around the corner

Storing the already buffered output/response into a corresponding item only for a failed step is what I currently think about.

An even more efficient approach could be to store the response only once the first time a step of a scenario failed.
For the time the scenario is in PROBLEM status, no further capture is stored.
A new capture of a failed step is only made when the web scenario previously recovered to OK status.