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 automatically is updated if a document is moved or deleted.
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
using <A HREF=..>, but also other resources being embedded or
linked by means of for instance <IMG> or <LINK>.
For absolute URIs, no checks are performed.