Of course there isn't any such thing. I thought I'd throw the question out and learn what others wear while they are writing.
I have a good friend who gets up at 4 a.m. everyday and begins writing in her pajamas and doesn't get dressed until she's through with her literary process.
I must confess I always get dressed before I start my day doing anything. Could be partly because this was what my mom did until she reached her 90s, then she'd eat breakfast in her bathrobe and get dressed afterwards. Another reason is for twenty-three years hubby and I ran a licensed residential care facility in our home for six adult developmentally disabled women. We never knew when licensing or the regional center might make a surprise visit and I didn't want to be caught looking less than professional.
I no longer worry about looking professional, but I always dress for whatever I'm going to be doing the rest of the day. Once I take my shower, I put on whatever I'm going to be wearing no matter what time that happens. I do not like to change clothes. Strange, but true. Hubby is just the opposite, he's wear his grungies until just before it's time to go somewhere.
Another reason I get dressed right away is because of family members who just drop in. No one bothers to knock around here and once we're up the front door is unlocked.
So... I could be writing in casual clothes because I'm staying home all day or you might find me in business type clothes because I have to go to a meeting--like today.
Tell me about your writing attire.