Or even fashion rules for that matter? I'm pretty sure they are two different things.
On Friday I wore an outfit to work I thought was alright as casual-Friday outfits go...black long-sleeved polo neck with a black faux leather T shirt on top...Light blue wash jeans. Open toe thin sandals with probably a 3inch heel.
My manager and more person did a double take and the receptionist pursed her lips and gave me a long cool stare. What cardinal fashion rules did I break? I am at a loss 
I've never had that reaction from people before. And this is a tech environment where the developers rock up to work daily in shorts and smelly T shirts.
Please tell me I looked jaw-droppingly awesome hence the reactions 
But seriously do you have any self imposed looking-good rules you follow at work? Would be good to know (so I can go ahead and break some rules of good taste) 