I inspect an element in google chrome and after modifying and saving my css file I reload the page to view the result.
Unfortunately my DOM tree is collapsed then.
Can it be achieved that it keeps expanded (and focused) at the same point?
You may try posting your feedback for this feature.
There's a link near the bottom of the page for posting bugs/feature requests: http://code.google.com/chrome/devtools/
It also says to mention keyword "DevTools" in your feature request
Didn't work for me. I'm guessing as of right now this is not possible. Firebug in Firefox seems to do it, though. You may want to try that.