Besides the senerio of two side by side usercontrols being cached, there is
another senerio, suppose one usercontrol uses embedded style(both menu and
treeview use these, and many other ajax controls also use this technique to
inject style definition into the html head element), and it contains a child
usercontrol who is also set to cached, and also uses embedded style, the two
controls expiration event will ussually not coincident, and now suppose that
the parent one is being regenerated while the child one has still a valid
cached version, in this case, since the child one's css settings will go
through the path of page.header.StyleSheet.CSSStyleString (in
BasePartialCachingControl.PreRenderRecursiveInternal), so it will not be
merged with the parent control's css setting, which results an incorrect css
setting string being stored into cache, so any subsequence output of the
parent usercontrol from cache will be incorrect.

Any comment or suggestions welcome.
JX