On My Yoast, everything inherits box-sizing: border-box while here it's the default content-box; this actually affects the size of the flex items. I'd consider to switch everything to border-box but for performance reasons this should be done at the root level using the universal selector * and not in the components.
Media queries breakpoints are hardcoded. On My Yoast instead, we're using some values from a config file. I'd consider to do the same thing here, maybe in a separate PR.
Copied from https://github.com/Yoast/yoast-components/pull/219#issuecomment-321549740