Compare this sample notebook in “old” Observable:
And in “new” Observable:
https://observablehq.com/d/1652c91181b0e2d9
The tex cell is centered with the text in “old” Observable, but is centered with the wider page in “new” Observable. I think in most cases the desire is more likely to be to center the equation with the text.
The issue seems to be that the CSS rule for max-width: 640px; doesn’t include .katex-display in “new” Observable.
Also, if you look at the second equation on the page, you will notice that it is slightly clipped at the top and bottom in “new” Observable. (At least it is in Chrome on Windows for me.)
The issue here seems to be overflow-y: hidden; on .katex-display in “new” Observable.