[csswg-drafts] [css-scroll-snap-1] Clarify which writing-mode is used for scroll-snap-align, scroll container or snap position element? (#3815)

hiikezoe has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-scroll-snap-1] Clarify which writing-mode is used for scroll-snap-align, scroll container or snap position element? ==
I've been thinking it's for sna-position element, but [jfkthame thinks it's for scroll container](https://phabricator.services.mozilla.com/D21628#inline-153048).

Here is an example that the writing-mode for snap position element differs from scroll container's value;
https://codepen.io/anon/pen/vMyRVV

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/3815 using your GitHub account