I'm sorry Reimu froze the game for you. This is an older demo of Dolmexica Infinite, so this issue should be fixed in a newer demo: https://captaindreamcast.itch.io/dolmexica-infinite
It would be interesting to know which version you encountered the freeze in. In the Dreamcast version, Reimu has some memory problems, because she is a rather large character, memory-wise. So there is a good chance the game will run into memory issues during loading. For reference, the Reimu vs King fights in story mode were pretty much at the upper limit as far as memory usage is concerned.
Of course memory issues shouldn't be a thing under web/windows, so if it crashes there, it was something else. Once again sorry you encountered that freeze.