Is there a solution that saves and analyzes data on a browser page?

Question

Browsers

Greetings, I will be grateful for advice on the implementation of the following task.
Given:
Through the browser, the user, entering the login and password, enters a page where there is some digital data that is variable and changes over time.
Required:
So that next time the browser automatically adds statistics for this data to the web page in brackets according to the specified algorithm. Access to the server that gives the web page is not and will not be.
What for?
For clarity when working with digital indicators of web services. I plan to use it for statistics systems, affiliate programs, advertising systems (targeting, context).
Example:
On the first visit to the web page, the variable looks like information X=5
On the second visit, it already looked like this: X=8 (+3)
Ideally, what is in brackets should be a complex formula that could involve variables such as the time that has elapsed since the last visit to the web page .
I myself think to look for browser extensions in the first place, perhaps such processing has already been implemented?
Thanks for answers!
UPDATE
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Additional options
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Answer 1

I do this on userscripts (GreaseMonkey). In addition to the always available localStorage and cookies, GreaseMonkey also has its own API for storing values ​​in the database (a separate database for each user script): GM_getValue/GM_setValue. On the plus side, after all localStorage cookies are cleared (against being tracked by scripts on the page), my userscript databases remain intact. You can keep any statics or store large objects (pictures in binary form, for example), the database on the SQLite engine allows this too.