We're currently appending px when using the .css() setter if the value given is numeric and the CSS property is not on the jQuery.cssNumber list.

From #2793 (comment):

This is a mess, we're adding more & more stuff to this array and it'll always be a problem as CSS is expanding. I'd like to get rid of this px-appending but it might be too big a breaking change.

This is probably too late for 3.0 but how about switching the blacklist to a whitelist? If jQuery doesn't append px for you it's easier to fix by yourself than if it does append erroneously. My guess would be 99% of the uses are common properties like (min-|max-|)(height|width) or top, left etc. We could identify those most popular ones and append px only to them. Migrate could restore the previous behavior but warn if someone passes a numeric value to properties that are not on the current blacklist.