Internet Explorer exists… How do you cope with that in combination with the CSS Grid? Most of my clients really want their websites to be working everywhere, because there’s still a (small, but big enough) percentage that uses IE.
Let these “small, but big enough” get rid of their conservatism. The solution to the problem is to make the check if the site has been visited via IE to display an alert that their browser is outdated and it would be time to install the normal browser)
There are people who simply can’t update their browser or install a new one, because their company won’t let them… Should we just forget about them?
Then don‘t use CSS grid. You can‘t use modern technology and expect it to work on outdated browsers.
As @mac_heibu mentioned, if your client has a large portion of their visitors using old (outdated) browsers, you’ll want to stay away from new (exciting) technology – like CSS Grid.
Quick question, what about the design requires you to use Grid? Is it just because it’s new? Did you already sell your client on it without explaining the limitations?
I’d bet with some creativity you can make something similar work with basic web elements.