The % width version should be fine in IE8.
I still get a tiny minority of IE8 visitors. I have stopped caring about them getting exactly the same look as modern browsers do. If the site displays and the content is readable, that’s enough as far as I’m concerned.
If they don’t see round-corners, shadows, css columns, or see standard inline-blocks in place of fancy flex-box layouts, it’s not the end of the world.
If you work too hard to accommodate them, they will never upgrade! 