> CSS grid > If you’re still using heavy-weight grid frameworks from UI toolkits like Bootstrap or Foundation, you are falling behind. This is a pretty sensationalist way to word this and honestly an absurd statement as a whole. If you have a public-facing site, ignoring everything that isn't up-to-date is a luxury you can't afford and blunt statements like these show ignorance in that regard.
CSS grid is supported in all modern browsers. Depending on your audience, there is a very good chance you can use it in favor of older and much less powerful and pleasant grid layout tools.
Developing for a b2b or enterprise product? That percentage is going to be lower. Developing for users outside advanced industrial countries? Lower.
And it's not like you can polyfill it or use a fallback -- CSS grid is a completely new way of building layouts that isn't amendable to automatic fallbacks, so you literally have to build an entirely different parallel way to do the same thing if you want those 12% to have the same experience. At that point... why bother using CSS grid at all?
The simple and sad fact is that until IE 10/11 fall totally out of use, grid is simply not worth learning for many projects.