Need feedback and help for survey page project

Need feedback and help for survey page project
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#1

Project Link - http://codepen.io/codejsp/pen/jypmrq

Recently found out about flexbox :grinning: so I tried recreating the survey page project.
I was wondering if my code looks correct (is there anything I could add/subtract/combine?).

Also just found out about the w3validator so I ran my code through that. I fixed most of the problems except I couldn’t figure out how to get the for attribute to work for my ul's. I’ve tried adding id's like I did to fix the other labels, didn’t work. So, I tried adding id's or classes where I thought they should be but still couldn’t come up with an answer.