Well, 'read' is an inadequate verb, as Homestuck has a soundtrack, and animation sometimes. And an intense and involved fan community, whose wild mass guessing is more or less key to some parts of Homestuck.
First among the things I learned is that foreshadowing can be more opaque the more people you can get to try to shed light on it. There is an extensive wiki for Homestuck and an exhaustive TVTropes page, which all add background dimension to the story. If it weren't for the foreshadowing being obviously there but not obviously leading to something, I wouldn't have gone to the wikis and - and this is key - I wouldn't be as invested in the story and the series.
Second is that you can baldly tell someone what is going to happen, in detail if you want to, and then proceed to tell the story that leads to the events you told them about, and, if you've done well, and gotten the reader involved in the story and made them care, they will still be shocked.
Other things I learned about writing from reading Homestuck I'll have to tell you later in another post.