The HTML in-browser version is prone to bugs, such as things failing to properly load and inconsistencies between it and the Windows version. Because of this being potentially extremely dependent on client-side browser issues and client-side internet issues, I'm afraid there is not very much I can do about this that I'm aware of.
If it doesn't end up working for you, I'd recommend downloading the executable instead! And if somehow neither works, you can always watch the video of the entire first chapter that is also downloadable!