https://drafts.csswg.org/css-writing-modes-3/#principal-flow

As a special case for handling HTML documents, if the root element has a body child element [HTML], the principal writing mode is instead taken from the values of writing-mode and direction on the first such child element instead of taken from the root element. Note that this does not affect the values of writing-mode or direction on the root element itself.

When applying writing-mode: vertical-rl to body, html has writing-mode value horizontal-tb (initial value) according to the note and there happens an orthogonal flow between html and body?

Each browsers behave differently about this:

• Edge: An orthogonal flow doesn't happen. body { writing-mode: vertical-rl } behaves as same as html { writing-mode: vertical-rl }.
• Chrome: It seems an orthogonal flow happens since the margin of body child elements doesn't collapse with the margin of a body element.
  • I filed https://bugs.chromium.org/p/chromium/issues/detail?id=845008 and it marked as WontFix.
• Firefox: Propagation of writing-mode is not supported. https://bugzilla.mozilla.org/show_bug.cgi?id=1102175

According to the old spec text, an orthogonal flow can't happen.

The propagated values are treated as if they were specified on the root element.