Single-line comment in HTML

From HTML comments:

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.

The definition of an SGML comment
is basically as follows:

A comment declaration starts
with <!, followed by
zero or more comments, followed by
>. A comment
starts and ends with
“--“, and does not
contain any occurrence of
“--“.

This
means that the following are all legal
SGML comments:

1. <!--
Hello -->
2. <!--
Hello -- -- Hello-->
3. <!---->
4. <!------ Hello
-->
5. <!>

Note that an “empty” comment tag, with
just “--” characters,
should always have a multiple of four
“-” characters to be
legal. (And yes,
<!> is also a legal
comment – it’s the empty comment).

Not all HTML parsers get this right.
For example, “<!------>
hello-->” 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.

There are two possible
reasons for this:

1. The
   browser sees the “>” character and
   thinks the comment ends there.
2. The
   browser sees the “-->”
   text and thinks the comment ends
   there.

There is also the
problem with the “--”
sequence. Some people have a habit of
using things like
“<!-------------->”
as separators in their source.
Unfortunately, in most cases, the
number of “-” characters
is not a multiple of four. This means
that a browser who tries to
get it right will actually get it
wrong here and actually hide
the rest of the document.

For
this reason, use the following simple
rule to compose valid and accepted
comments:

An HTML
comment begins with
“<!--“, ends with
“-->” and does not
contain “--” or
“>” anywhere in the
comment.