IE10 and IE11 support Grid, albeit a slightly different syntax to an older spec. CSS Grid is not new. IE support was there long before the other browsers. You don't need to write two entirely separate layouts to use Grid. You don't need a polyfill.

Jen Simmons made a really good video about exactly this topic - https://www.youtube.com/watch?v=7msERxu7ivg

Rachel Andrews' blog post about IE and Grid - https://www.rachelandrew.co.uk/archives/2016/11/26/should-i-...

Mozilla article about progressive enhancement and Grid - https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_La...