So I had an idea for two column form layout using grid with fieldset using subgrid.
It works perfectly in Firefox... but not in Edge or Chrome or...
So I checked "canIuse" and it said support was everywhere. But then I check in F12 developer tools in Edge and it says "not supported in ... [long list of browsers]".
So I decide to go for "@supports"... and it is still broken.
So...
Either a) "canIuse.com" is lying and a long list of browsers that it says support subgrid do not
Or b) @supports is lying and a long list of browsers that it reports as supporting subgrid do not
As it happens, I have an acceptable workaround that would work if @supports told the truth.