When authoring HTML in standards mode, incorrectly formed comments can break your pages, resulting in part or all of your content being commented out. When authoring XHTML or XML, incorrect comments will result in your documents not being able to be displayed at all.
HTML 5
From W3C HTML 5 Recommendation Section 8.1.6:
Comments must start with the four character sequence U+003C LESS-THAN SIGN, U+0021 EXCLAMATION MARK, U+002D HYPHEN-MINUS, U+002D HYPHEN-MINUS (<!--). Following this sequence, the comment may have text, with the additional restriction that the text must not start with a single ">" (U+003E) character, nor start with a U+002D HYPHEN-MINUS character (-) followed by a ">" (U+003E) character, nor contain two consecutive U+002D HYPHEN-MINUS characters (--), nor end with a U+002D HYPHEN-MINUS character (-). Finally, the comment must be ended by the three character sequence U+002D HYPHEN-MINUS, U+002D HYPHEN-MINUS, U+003E GREATER-THAN SIGN (-->).
Example
The following is a valid comment in HTML 5.
<!-- valid html comment -->
HTML 4.01
From W3C HTML 4.01 Recommendation Section 3.2.4 - Comments:
White space is not permitted between the markup declaration open delimiter ("<!") and the comment open delimiter ("--"), but is permitted between the comment close delimiter ("--") and the markup declaration close delimiter (">"). A common error is to include a string of hyphens ("---") within a comment. Authors should avoid putting two or more adjacent hyphens inside comments.
Examples
The following are valid comments in HTML 4.01.
<!-- valid html comment --> <!-- valid html comment -- >
XML
From the W3C XML 1.0 Recommendation Section 2.5 - Comments:
Definition: Comments may appear anywhere in a document outside other markup; in addition, they may appear within the document type declaration at places allowed by the grammar. They are not part of the document's character data; an XML processor may, but need not, make it possible for an application to retrieve the text of comments. For compatibility, the string "--" (double-hyphen) must not occur within comments. Parameter entity references must not be recognized within comments.
[15] Comment ::= '<!--' ((Char - '-') | ('-' (Char - '-')))* '-->'
As described in the grammar for comments in XML, a comment has the form
<!-- characters excluding adjacent dashes -->
Examples
The following is a valid comment in XML and XHTML.
<!-- valid xml/xhtml comment -->
The following are not valid comments in XML and XHTML.
<!-- invalid -- xml comment --> <!-- invalid xml comment ---> <!-- invalid xml comment -- >