CSS Grid is a wonderfully powerful technology, making possible incredibly complex and interesting layouts with fractions of the effort of older tools. But with great power comes great responsibility, for it is now also very easy to shoot the accessibility of your site in the foot.
Author Rachel Andrew:
I think this is something we sorely need to address at a CSS level. We need to provide a way to allow the tab and reading order to follow the visual order. Source order is a good default, if you are taking advantage of normal flow, a lot of the time following the source is exactly what you want. However not always, not at every breakpoint. If we don’t give people a solution for this, we will end up with a mess.