it should also be possible to add an ID
and runat="server"
to your fieldset and control visibility through code-behind. Just remember to write 'ID' in upper letters.
<fieldset ID="myFieldset" runat="server">
You won't be able to control the legend text, unless you give it an ID and runat itself. But visibility is absolutely possible.
The upside of this approach is: no needless html markup (Panel would be extra div).
The downside: fieldsets are not really asp-controls, so some things might give you exceptions, so use carefully.
I use this approach only when I want to prevent controls from rendering at all in certain cases (visibility does that).