from http://www.w3.org/TR/xhtml1/#guidelines
C.4. Embedded Style Sheets and Scripts

Use external style sheets if your style sheet uses < or & or ]]> or --. Use external scripts if your script uses < or & or ]]> or --. Note that XML parsers are permitted to silently remove the contents of comments. Therefore, the historical practice of "hiding" scripts and style sheets within "comments" to make the documents backward compatible is likely to not work as expected in XML-based user agents.
Leaving aside the fact that

Use external style sheets if your style sheet uses < or & or ]]> or --. Use external scripts if your script uses < or & or ]]> or --.
means nothing to me, what opinions do you have about the current practice of using <!-- --> comment tags to hide embedded styles and javascript in your (X)HTML? This is something I have been using because it is meant to prevent your internal code being displayed on older browsers, but the W3C are now advising against this.

Regards,
Andy.