I had a need want to have my field groups formatted in fieldsets with legends rather than divs with labels as I believe the former is more semantically useful (ie I know that a fieldset is for grouping similar fields, but I don't know what a div groups).
I would have hoped that I would be able to achieve this with a .ss file, but it seems that /sapphire/forms/FieldGroup.php returns HTML directly using the function FieldHolder().
I was able to adjust the HTML code there to get the output I desired, but why isn't it in a template?
It is not in a template as we found early on having each form field in its own template caused a large performance lost, so as a workaround for that its in the php. You could use your own custom subclass of the field to override the template it outputs
I needed to do this for a form which renders dynamic fieldsets from a db query and ended up extending FieldGroup, here is the code if anyone is interested. Very straight-forward and quite specific (needed a table layout) but you can do whatever you want: