First of all, thanks to all of the new players joining Super Markup Man during the Remote Play Together sale! This sale does highlight something that has been a sticking point for many players, though. Historically, you were required to finish the single-player HTML mode first before you could play co-op in CSS Mode. For some players, HTML Mode was also just not challenging enough to play all the way through.
So... I've removed this restriction, meaning CSS Mode is now available from the very start! Of course, it's still recommended to play HTML Mode first if you are not familiar with HTML syntax. Free Play Mode will also remain locked until you complete CSS Mode, as it wouldn't make sense to jump straight into such an unstructured environment.