Neat. Such services ("delta filters" in information filtering, a sub-part of information retrieval, "diff alerts" in Web mining, another sub-part of information retrieval) are useful to implement anything from price comparison to legal canaries (cf. https://en.wikipedia.org/wiki/Warrant_canary).

It would be useful to have a language to specify exactly what kind of event triggers an alert (e.g. regex, presence/absence of keyword or phrase, or domain from SERP).