Comments are an SGML construction, and are normally only valid inside a DTD. However, as 節 3.4, “Escaping back to SGML” shows, it is possible to use SGML syntax within your document.
The delimiter for SGML comments is the string
“--
”. The first occurrence of this string
opens a comment, and the second closes it.
If you have used HTML before you may have been shown different rules
for comments. In particular, you may think that the string
<!--
opens a comment, and it is only closed by
-->
.
This is not the case. A lot of web browsers have broken HTML parsers, and will accept that as valid. However, the SGML parsers used by the Documentation Project are much stricter, and will reject documents that make that error.
The SGML parser will treat this as though it were actually:
This is not valid SGML, and may give confusing error messages.
As the example suggests, do not write comments like that.
That is a (slightly) better approach, but it still potentially confusing to people new to SGML.
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