Responsive 3 column preview(html, css)

This wasn't too much of a challenge for me overall, I think it only took me about an afternoon to complete, but pretty much other than the centering being a little bit off on the desktop version, I don't think there's much of a problem, it wouldn't be too much trouble for me to fix that by any means. I'm also not sure if this or my other projects I've been completing are completely optimized for all browsers, I've only really checked how it appears on Microsoft Edge and Chrome, but I remember checking my qr component to see that it was pretty poorly optimized for Internet Explorer- other than that I'm not sure how this would look on Opera, Firefox or Safari for example. So my question would be, what would you recommend for making sure this is well optimized for all other platforms?