This site uses localStorage to remember what posts have changed. That means that if:
Something erases your localStorage on this site
You use a different browser
You use a different device or computer
Then your history will lost. This is the only way to do it without creating a server and
making you create an account and log in.
There is no way (outside of keeping a history) for an external site (or user) to know when a
Until a history is established, results are sorted by the last activity time.
This site uses the Stack Exchange API,
which limits the number of calls per minute and per day.
This means that you cannot poll here faster than once every 60 seconds, and only about 1,000,000
individual posts can be polled per day.
So, for example, if you have 750 answers and 50 questions, that's 800 posts. If you poll once
every 60 seconds, then you will run out of API quota in 20 hours.
Other factors can affect quota, like other machines on the same IP, multiple tabs running, etc.