1. Requirements: the page first renders a pile of data (old values), then returns changed data (new values) every 3 seconds, and re-renders the corresponding data on the page (the old values will be overwritten by new values).
2. Problem: if the newly returned value is greater than the old value, an upward arrow appears and disappears after 5 seconds; otherwise, the arrow is down;
3. There are thousands of values on the page. The new values that change every 3 seconds will not return too many, ranging from 20 to 1.
