plugin dev – minimum base css specificity?

There’s no standard. CSS specificity in themes varies wildly, and I’m convinced many theme authors are not even aware of the concept given some of the CSS I’ve seen.

If something is very important to the appearance of your plugin, then high specificity may be desirable, but in many cases if you’re just styling divs with unique class names, or form elements, then you should keep specificity low so than it can inherit styles from the theme as appropriate.

So really it’s case by case, and you’re not going to be able to guarantee results with all themes. A good rule of thumb is that your specificity should reflect how important it is that the styles are respected.