Ask HN: How to respond to “whats the biggest technical challenge you've solved?”

I'm curious how people respond to this question, especially front end developers. I've been interviewing and struggling with answers for this. It's not that I have never had challenges but many of them have been very craft centric such as "this has to look and feel smooth on many different devices." And a lot of time the challenge is "this needs to be done quickly by this date" or "we don't have any tests so can you figure out how this API should work, document that and add test coverage."

I tried asking myself how would my colleagues answer this question about their work and I also can't answer so I don't think it's just a matter of me not taking on challenging work.

I think I just lack any context or framework for what would make a good answer.