From what I’ve gathered using images found online, the yard markings in most National Football League stadiums appear to be set in a Clarendon Bold or Black, or some modification thereof. (The zero, for example, is wider than you’ll see in most fonts to balance it with the width of the number to its left.)
It’s not clear when this standard was set and how strict the NFL is about it. There is no mention of the yard markings in the online rule book. I welcome football fans to weigh in with their knowledge on the subject.
According to official NFL rules, 30-foot deep scoring end zones bookend the field, which is demarcated by horizontal lines every five yards, with two-yard-long numbers indicating yard lines in multiples of 10 placed exactly twelve yards in from the sideline. Their font, surprisingly, isn’t officially standardized.
Update, Feb. 7, 2016: During 2015, to celebrate Super Bowl 50, every stadium had a gold 50-yard line marker. The style was also used today for the Super Bowl 50 field itself. These numbers appear to be based on a Century, not Clarendon.