Wiki reads from many sites but writes only to the site where you begin each browser tab, the origin, where the javascript comes from.
You will want to login to the origin before you start writing. If you own many sites you will find you use many tabs and login on all of them.
If you write on a site you don't own then your changes will be stored in the browser local storage and will be there when you return. See Local Changes
If you forget to login, or were logged out without noticing, you can login to a site you own and 'fork' your local changes to your site.
If you lose connectivity to your site then your edits will be saved locally until you can reconnect and 'fork' your local changes to your site.