The important thing is that nowadays, whether a particular browser supports JavaScript or not is irrelevant (clearly the great majority do) - it is irrelevant because almost all understand script blocks, which means that they know to ignore the JavaScript even if they can't interpret it.
Matt Kruse gives a slightly more detailed explanation on his JavaScript Toolbox site for why specifically not to use HTML comments within script blocks.
Quoted from that page:
Don't Use HTML Comments In Script Blocks
In the ancient days of javascript (1995), some browsers like Netscape 1.0 didn't have any support or knowledge of the script tag. So when javascript was first released, a technique was needed to hide the code from older browsers so they wouldn't show it as text in the page. The 'hack' was to use HTML comments within the script block to hide the code.
Using HTML Comments In Script Is Bad
// DON'T do this! Code is just representative on how things were done
<script language="javascript">
<!--
// code here
//-->
</script>
No browsers in common use today are ignorant of the <script> tag, so hiding of javascript source is no longer necessary. In fact, it can be considered harmful for the following reasons:
- Within XHTML documents, the source will actually be hidden from all browsers and rendered useless
- -- is not allowed within HTML comments, so any decrement operations in script are invalid
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