>>One of the features of the application (that will be running on the tablet) is that user, every week or so, will create a list of XML files. E.g PM_123.XML, PM_ABC.XML, and so on. I need to 'tag' these XML files as belonging to this particular tablet (not user, because different users may log into this tablet at different times). So my idea is to create an XML file that will "enclose" the XML files created by user/tablet. For example:
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>Have not checked on iOS, but on Android another user to same physical tablet will have his own space for data similar to "users" in windows. So different user should have different "deviceID" and separate localStorage / IndexDB.

> Without any kind of app even downloaded files will land into user-specific download areas.

Not 100% sure but I don't think that applies to Browser localStorage - it's stored by domain name so there's no way of making localStorage user specific even if you wanted to....