One of the most annoying thing within creating HTML pages is to find out if all your links work. Concepts superior to HTML and HTTP usually maintain a database, which is automatically updated if a document is moved or deleted. (And they already did that in they seventies.)
For local URIs (those which only refer to files), hsc checks existence of documents in your HTML project. When you refer to a non-existing local URI within your source, hsc will view message #21.
This can not only be caused by documents which are referred to
<A HREF=..>, but also other resources being embedded or
linked by means of for instance
For absolute URIs, no checks are performed.