About
Comments may appear anywhere in a document outside other markup.
They are not part of the document's character data
Example
<!-- declarations for <head> & <body> -->
Note that the grammar does not allow a comment ending in —>. The following example is not well-formed.
<!-- B+, B, or B--->
Support
Error: Double hyphen within comment
For compatibility, the string – (double-hyphen) must not occur within comments..