It's a neat demo of CSS Grid, for sure, but the above would be my much preferred way of doing this kind of layout. Maybe if there were a need for elements (quotes, callouts, etc.) in the side columns alongside the content, then it makes more sense to use Grid.
i think your version will break a bunch of common uses of `margin-{left,right}`; e.g. indenting like
blockquote {
margin-left: 15px;
/* ... */
}
won't work properly (it'll go all the way to the left). i haven't checked, but i think that in the grid version, you'd get an indent as expected.
and i think that's the grid version's main advantage – it doesn't "use up" `margin` and `max-width`, so you're free to use them to style stuff that appears in the body. feels a bit more composable, though at the cost of also feeling massively overcomplicated :/
I left out some assumptions for brevity, but my presumption was that everything inside of the container (like p tags) would be display: block, so it would try to fill the max-width of 65ch with blank space anyway.