The New CSS Layout
Use auto as your flex-basis and, if you’ve set a width on the item, that width will be used as the flex-basis value. If you haven’t set a width, auto resolves to the content size.
Perhaps the client has had a bad experience in the past, with customers complaining th at the site doesn’t work for them. Put their mind at ease by explaining how, by serving a simpler layout to older browsers, they are less likely to run into these issues. Visitors with older browsers will get a fast and performant website, tailored to the device they have. That’s a far better proposition than trying to somehow create a modern experience using old technology, something that is more likely to cause them problems.