I have an Array question where I'm hiding some fields by using CSS in template, but my array is mandatory, and so participant cannot proceed to next question because of hidden fields.

Any way to make some fields in an array mandatory (or at least, negate the ones that I have hidden from the mandatory evaluation)?

No. A workaround would be to add a hidden "N/A" column and toggle that with JavaScript where necessary.

That's an interesting idea. So what I would do is:

1) Hide the fields (cells) I want to hide
2) Add the No Answer option to the question
3) Show No Answer only on the rows where I've hidden the cells

You say I can " toggle that with JavaScript" -- can you explain, show what you mean? Is there an example LSS somewhere that might demonstrate this?

I did not say "Add the No Answer option to the question", I said to add another sub-question for N/A.