According to <http://webkit.org/blog/148/web-inspector-update/>, I should be able to edit CSS in WebKit nightlies. It does not appear to work. I have tried on Tiger and Leopard, using r28505. The inspector appears, and I can see CSS, but I can't edit it (just like other recent nightlies).
To edit CSS you need to double click. It also only works on rules not computes style.
My guess is that Mac is running into bug 16349.
Timothy and Adam, you're both correct. It seems that I had tried to edit the computed style as well as a couple of properties under user agent stylesheet, and these are the only immutable areas. Should I file an ER requesting the ability to edit arbitrary properties of arbitrary elements / classes, or is that already on the roadmap?
I think this is a dupe of bug 16349 then. *** This bug has been marked as a duplicate of 16349 ***
you can only edit author styles, not user agent style, mapped attribute style or computed styles. bug 16349 tracks making this better.