Comment-based syntax typically uses XML-like comments or special preprocessed comment-like elements to either annotate parts of a Web page (see StructureAnnotation) or to introduce program code into the document (see PythonInWebPage).
See PythonInWebPage for an example using JSP-like syntax. Here is a StructureAnnotation version using HTML-like syntax:
<table> <!-- loop:item in items --> <tr> <th>Name</th> <td><!-- expr:item.name --></td> </tr> <!-- loop:end --> </table>
One potential advantage of comment-based syntax is that HTML-aware tools can ignore the comments, should the syntax have been chosen correctly.