how to show or hide a field set using a button event

I have a field set - id = idFldInstructions. I have used css to make that invisible on load. I then have a button with an event 'Click' set to show that field set.
Click - Show #idFldInstructions
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I can get it to work to show a control within the field set but not the field set itself. Should that be possible? If not are there 'areas' that I can use as a container to show and hide content??
Regards
Nick

Thanks - I will try that later. Must clean up broken branches first😟
Don't quite understand hidden class. Isn't that the same as using

div{
  visibility: hidden !important;
}

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I shall try it and see.

No.

Use a container instead, with the fieldset inside it.

That is the solution plus the 'hidden' class - thanks.