element existence selector

Since CSS2 we can express the "existence of attributes",

   http://www.w3.org/TR/WD-CSS2/selector.html#h-5.7.1

with the *[att]* selector,

*    element[att] {do}*

and it is easy implement this kind of conditional selector in CSS parsers...

The suggestion (or question) here is about use *the same ideia with
elements*, a kind of existence operator,

*    e1 <existenceOperator> e2 {do}*

to say "if element *e1* exists, selects the *e2* element".


* if it is a new suggestion, can I discuss here this subject?

* if there are *some old discussion about it*, sorry, can somebody reply
with a link?

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PS: I think that this kind of <*existenceOperator> *can used in a big
subset of (imagined) applications of the problematic
previous-sibling-selector, and also used as a "*trigger*" in event
selectors, like
      e1:hover <*existenceOperator*> e2 {do}