In Florence this past weekend and the sign was clearly posted on the entrance to the Il Duomo Cathedral regarding the dos and donts--international symbols with the line through them (i.e. no food, no smoking, no shorts, etc.). I absentmindedly walked in with shorts on and wasn't even given a second look.

It was on a Saturday and there were many tourists there with shorts on. I didn't really take notice of any sleeveless tops, etc. I'm not sure why they didn't say anything to me or anyone else---my only guess is that it was a crowded, touristy Saturday and they just chalked it up to that?