Neither. It would be a website you create with calibre-server version-new-and-improved.
Works on any computer, tablet, smartphone, browser, embedded device (if powerful enough
), etc.
No installed app, program, or plugin necessary, though you will have to allow the website to use localstorage or rely on constant internet connection.
See the aforementioned OverDrive Read, or Kindle Cloud Viewer, for examples of such usage.