Yes, if you center vertically with a flex-box, and you aren't doing anything weird with margins or overflow, and that's the only thing you do with it, you'll ge

Yes, if you center vertically with a flex-box, and you aren't doing anything weird with margins or overflow, and that's the only thing you do with it, you'll get non-centered results on IE8-9. Not a problem. But if you're only looking for that, then it seems very churlish not to use table layout, since that is the same amount of CSS and works in IE8+. There's no excuse for breaking layout in a browser if the alternative is just as simple and has no downsides. Once you need to use flexboxes for more than vertical centering, it can cause more problems. Using it in place of inline-blocks, for example (one of its useful applications), can leave your content very weirdly laid out on older IEs.