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<TITLE>HTML comments</TITLE>
<META NAME="description" CONTENT="An explanation of what HTML comments are.">
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<META NAME="author" CONTENT="Arnoud Engelfriet">
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<H1 ALIGN=CENTER>HTML comments</H1>
Since HTML is officially an SGML application, the comment syntax used
in HTML documents is actually the SGML comment syntax. Unfortunately
this syntax is a bit unclear at first.
<P>
The definition of an SGML comment is basically as follows:
<BLOCKQUOTE>
A <EM>comment declaration</EM> starts with <CODE><!</CODE>, followed by zero or
more comments, followed by <CODE>></CODE>. A <EM>comment</EM> starts
and ends with "<CODE>--</CODE>", and does not contain any occurrence of
"<CODE>--</CODE>".
</BLOCKQUOTE>
This means that the following are all legal SGML comments:
<OL>
<LI><CODE><!-- Hello --></CODE>
<LI><CODE><!-- Hello -- -- Hello--></CODE>
<LI><CODE><!----></CODE>
<LI><CODE><!------ Hello --></CODE>
<LI><CODE><!></CODE>
</OL>
Note that an "empty" comment tag, with just "<CODE>--</CODE>" characters, should
always have a multiple of four "<CODE>-</CODE>" characters to be legal.
(And yes, <CODE><!></CODE> is also a legal comment - it's the empty comment).
<P>
Not all HTML parsers get this right. For example, "<CODE><!------>
hello--></CODE>" is a legal comment, as you can verify with the
rule above. It is a comment tag with two comments; the first is empty
and the second one contains "> hello". If you try it in a browser,
you will find that the text is displayed on screen.
<P>
There are two possible reasons for this:
<OL>
<LI>The browser sees the ">" character and thinks the comment ends
there.
<LI>The browser sees the "<CODE>--></CODE>" text and thinks the comment ends there.
</OL>
There is also the problem with the "<CODE>--</CODE>" sequence. Some people have a
habit of using things like "<CODE><!--------------></CODE>" as
separators in
their source. Unfortunately, in most cases, the number of "<CODE>-</CODE>"
characters
is not a multiple of four. This means that a browser who <EM>tries</EM>
to get it right will actually get it <EM>wrong</EM> here and actually
hide the rest of the document.
<P>
For this reason, use the following simple rule to compose valid and
accepted comments:
<BLOCKQUOTE>
An HTML comment begins with "<CODE><!--</CODE>", ends with
"<CODE>--></CODE>" and does not contain "<CODE>--</CODE>" or "<CODE>></CODE>" anywhere in
the comment.
</BLOCKQUOTE>
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Copyright © 1997 <A HREF="http://www.stack.nl/%7Egalactus/">Arnoud "Galactus" Engelfriet</A>.
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