# 一千萬個為什麽

## 最佳答案

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 -->
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.