Comments are used to explain your code and may help someone when they are editing the code at a later date. Problem, comments are different in each language. I guess if you stuck with /*…*/ you would hit the majority.
CSS: /*This is a comment*/
HTML: <!--This is a comment-->
JavaScript: // This is a comment
JavaScript: /* This is a comment */
C#: // This is a comment
C#: /* This is a comment */
TSQL: -- This is a comment
TSQL: /* This is a comment */