How it works
Anyone can fetch a page twice and compare the bytes. That product reports a change every hour on almost every page on the internet. What makes a change detector worth paying for is what it throws away first.
A real browser loads the URL and runs its JavaScript, so a page that builds itself client-side is read the way a person sees it. Nothing is served from cache — "what does this page say right now" is the entire product.
Nothing, a CSS selector, or a field map. With no selector you get the page's main content, which already excludes navigation, footers, asides and advertising. A selector or field map switches to the whole document so it can reach anything.
Whitespace, wrapping and invisible spaces, always. Then the volatile classes you left on — timestamps, relative times, tokens, cache-busters. This is the step that decides whether an alert ever means anything.
sha256 over the normalised bytes. Equal means no change; different means change. Nothing is inferred and no model is consulted — you can re-derive the verdict from the content we hand back.
A line diff of what was added and removed, or, in field mode, the named values that changed with their before and after. The diff is evidence for a human, not the verdict.
Step three, in full
timestampson by defaultDates and clock times, in ISO, numeric and written forms.
relative_timeson by default"3 minutes ago", "just now", "yesterday".
tokenson by defaultUUIDs, CSRF and nonce values, and long opaque hashes such as asset fingerprints.
cache_busterson by defaultVersioning query parameters on URLs (?v=, ?t=, ?cb=, ?hash=).
numbersoff by defaultEvery number on the page. Off by default — a number is usually the answer.
You can also strip literal substrings of your own with ignore. Literal, not regex — a caller-supplied regular expression is a denial-of-service waiting to happen and a debugging problem nobody wants at 3am.
The rail split
A one-off check has one payer and one answer, so it fits on any rail. A monitor runs hourly, daily, weekly and has to draw 3 credits from somewhere on every pass, then deliver the result somewhere. A keyless caller has neither a renewable wallet nor an address. We would rather tell you that before you pay than after.