from http://htmlhelp.com/reference/wilbur/misc/comment.html
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 withThis means that the following are all legal SGML comments:<!, followed by zero or more comments, followed by>. A comment starts and ends with "--", and does not contain any occurrence of "--".Note that an "empty" comment tag, with just "
<!-- Hello --><!-- Hello -- -- Hello--><!----><!------ Hello --><!>--" 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:
There is also the problem with the "
- The browser sees the ">" character and thinks the comment ends there.
- The browser sees the "
-->" text and thinks the comment ends there.--" 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.
No, you have to close the comment with -->.
Let's keep it simple. Loved @digitaldreamer 's answer but it might leave the beginners confused. So, I am going to try and simplify it.
The only HTML comment is <!-- --> It can be used as a single line comment or double, it is really up to the developer.
So, an HTML comment starts with <!-- and ends with -->. It is really that simple. You should not use any other format, to avoid any compatibility issue even if the comment format is legit or not.
No, <!-- ... --> is the only comment syntax in HTML.
TL;DR For conforming browsers, yes; but there are no conforming browsers, so no.
According to the HTML 4 specification, <!------> hello--> is a perfectly valid comment. However, I've not found a browser which implements this correctly (i.e. per the specification) due to developers not knowing, nor following, the standards (as digitaldreamer pointed out).
You can find the definition of a comment for HTML4 on the w3c's website: http://www.w3.org/TR/html4/intro/sgmltut.html#h-3.2.4
Another thing that many browsers get wrong is that -- > closes a comment just like -->.
Source: Stackoverflow.com