You must have played right after I uploaded the first time, and you must have clicked the full-screen icon that Unity puts on all the WebGL builds by default. It wasn't my intention to allow full-screen, so I removed it from the html. It was literally just a few minutes before I saw your comment. When played at the default resolution as intended, the boundaries are correct. Thanks for the feedback.
Update: From one of the other comments I'm guessing you were talking about vertical movement. That is indeed a valid criticism. Initially I was going to have some obstacles to mess around with in that area but that didn't end up happening.