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 by default. When you refer to a
non-existing local URI within your source, hsc will print
message #21. This can not only be caused by documents which are referred to
<A HREF=...>, but also other resources being embedded or
linked to by means of
Since V0.932, hsc has been able to check external URIs as well, as long as your OS provides a basic socket API (the only notable exception being a plain AmigaOS 3.1) and the necessary connectivity. As this will considerably slow down the translation, it is off by default and can be enabled by means of the CHECKEXTERNAL option.
For checking external URIs, see also the notes on proxy-related environment variables.