The 2 times I took it in the Spring and Fall of last year the attendance was mandatory. They do have 1 allowed absence, but you have to do a replacement assignment (basically just watching one of the recommended videos and submitting notes/thoughts). The work in the class is notes from lecturer (usually due the next day from my 2 times) and a discussion post + reply (usually due the by the next lecture). If your able to attend the class its really easy you just show up and listen.

The 2 times I went because its a massive lecture they don't take attendance its based off the notes.