The : hover is the part of the selector, not CSS rules.
Inline styles only have rules. the selector is implicit to be the current element.
The selector is an expressive language that describes a set of criteria to match elements in an XML-like document.
However, you can get close, because a style set can technically go almost anywhere:
<html>
<style>
#uniqueid:hover {do:something;}
</style>
<a id="uniqueid">hello</a>
</html>