Developers probably cannot use App Inventor to 'track changes' on a Web page.
A developer could use this awkward method (for various reasons it is impractical)
- download the web page source (scrape the web page How can I get Source(html) from WebViewer) at time #1
- download the web page source at time #2
- compare the two downloads and look for differences
Another way might be to use the when WebViewer1.WebViewStringChange value at successive times. If the values change between the times, a change has possible occurred on the Web page. What change? You have to determine that by inspecting both value See Taifuns tutorial http://puravidaapps.com/snippets.php#2webviewstring
Someone might have other ideas. What you expect to do is very complex to be able to detect specific changes on a web page (i.e. a new image, new text etc.).
What tools you can use w.r.t. the Web page are: