But my supervisor is not happy with that. His idea is HTML5 infinitely less reliable than Javascript if the user's browser doesn't understand HTML5.
I think he is not familiar with HTML5 more than he thinks its incompatibilities
HTML5 is still not widely used but it is the future.
any other client side validation techniques???
JavaScript is the best (alternative is VB script, but it is similar).
With Java Applets (which is a kind of a Java application that downloads to browser and execute) you can validate from Java code.
Since AJAX, jQuery like JavaScript based technologies are very popular, you will find less people who disabled JavaScript on browsers.