Local files are suitable for loading/saving files from other mindmapping applications.
Export your design as a PNG image.
Your browser's localStorage can handle many mind maps and creates a permalink, but this URL cannot be shared.
Firebase offers real-time synchronization for true multi-user collaboration.
Use this to access a generic DAV-like REST API.
Maps stored in Google Drive have a permalink URL and can be shared with other users, if you allow this by setting proper permissions (inside Google Drive itself).