Html
<!-- comment here -->

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Css
/*comment here*/

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Javascript
<!-- comment 1 here
javascript code here
// comment 2 here -->
Without the <!-- and -->, the browser will display "comment here 1" as normal text. So if it is html document, you need to put <!-- and -->, if it is js document, you don't need to put <!-- and -->.

The two slash // tell the browser's JavaScript interpreter that the rest of the line is not valid JavaScript code.

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Ajax
// comment here

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Php
1) // comment here
2) # comment here
3) /* comment here */

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Mysql
/* comment here */

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Asp
'comment here