Web Inspector (Safari & Chrome): A/B testing changes? Is there any technique for A/B testing CSS changes made to an element in Web Inspector?

That is, evaluating before &amp; after states (loaded vs changed) for a given element.
You can do a sort of this if all your changes live in the same external stylesheet. Once you are done with the CSS tweaking, switch to the Resources panel and find the affected stylesheet resource. It should have a subtree of revisions beneath. The topmost revision is the current one, the deepest one, called "(original)", is the original resource version. Right-click any revision and choose "Revert to this revision" in the popup menu (the original version in your case). The stylesheet will get reverted and the revision with the changes will become the second-topmost one. You can revert to this revision once again to re-apply your CSS changes.