JDK6 and JDK7 sources has broken link in src/share/classes/java/io/package.html to
<a href="../../../guide/serialization">Serialization Enhancements</a>
I've checked that it's also visible here:
http://download.java.net/jdk7/docs/api/java/io/package-summary.html
and here:
http://java.sun.com/javase/6/docs/api/java/io/package-summary.html