[css-writing-modes] body propagation of writing-mode/direction/text-orientation and computed vs used styles #12586
Description
The current Writing Mode spec defines writing mode propagation like this:
As a special case for handling HTML documents, if the root element has a body child element [HTML], the used value of the of writing-mode and direction properties on root element are taken from the computed writing-mode and direction of the first such child element instead of from the root element’s own values. The UA may also propagate the value of text-orientation in this manner. Note that this does not affect the computed values of writing-mode, direction, or text-orientation of the root element itself.
IIRC the reason we made this only affect the used style on <body> was because it created difficulties in implementations to affect the computed style, not because it's better. (AFAICT, it's actually better for authors if it does affect the computed style.) However WebKit has the opposite problem: it's much more difficult to not affect the computed style.
Checking the computed style here is pretty weird edge case, so my question here is: do we need interoperability on exactly how the <body> style gets propagated here, or can we allow both options?