If you're a Counter-Strike professional who has a particular soft spot for Crocs, we have some bad news for you. As noted in the recent update to the ESL Pro Tour rulebook, the ESL has specifically targeted the footwear and outlawed them from being worn by players at official ESL events.
As mentioned in a clarification for article 4.3 Clothing, the ESL states, "Crocs are considered open shoes and therefore not allowed."
The ESL goes on to add, "The players and teams need to ensure that they are all in equal coloured team attire, long trousers and closed shoes (i.e. shorts, flip-flops and (i.e. shorts, flip-flops and foam clogs e.g. crocs clogs are not allowed). Any kind of headwear is forbidden. Penalties will be awarded for minor violations of this rule (a minimum of $250 fine), but in major cases (for example offensive content, other team branded clothing etc.), players will not be allowed to start their matches before the problematic piece of clothing has been replaced."
This rule change is just one of many, as the ESL has also addressed a bunch of other areas, including game specific rules, roster restrictions rules, replacement rules, and more.