Play crossword with friends at https://xw.carlwa.com.
CrossWordy is similar to downforacross.com but you need to upload your own puzzle.
You can open the HTML file directly i.e. ~/code/CrossWordy/index.html?id=005M6KfPJZybaIoGCRA3
You can view an existing puzzle i.e. ~/code/CrossWordy/index.html?id=005M6KfPJZybaIoGCRA3
This uses the production FireStore database, but users don't have permission to cause any harm.