To give some context; in our initial implementation we supported only block/inline ScrollDirection, as that was considered sufficient for the AnimationWorklet Origin Trial.

Notably the spec also has an issue to consider removing the physical directions.

As per surma@, we would like to support horizontal/vertical in order to make ScrollTimeline more understandable to web developers. So let's do that.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/73b31c297c48b99f2d352625b74f159ef33e0f91

commit 73b31c297c48b99f2d352625b74f159ef33e0f91
Author: Stephen McGruer <smcgruer@chromium.org>
Date: Tue Sep 18 15:49:35 2018

Support horizontal/vertical orientation for ScrollTimeline

Bug:  834314 
Change-Id: Ib4fc26eea22bf8a84e2810dbf20f05e552f1fe18
Reviewed-on: https://chromium-review.googlesource.com/1228940
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592054}
[modify] https://crrev.com/73b31c297c48b99f2d352625b74f159ef33e0f91/third_party/WebKit/LayoutTests/fast/animation/scroll-animations/scrolltimeline-currenttime.html
[modify] https://crrev.com/73b31c297c48b99f2d352625b74f159ef33e0f91/third_party/blink/renderer/core/animation/scroll_timeline.cc
[modify] https://crrev.com/73b31c297c48b99f2d352625b74f159ef33e0f91/third_party/blink/renderer/core/animation/scroll_timeline.h
[modify] https://crrev.com/73b31c297c48b99f2d352625b74f159ef33e0f91/third_party/blink/renderer/core/animation/scroll_timeline_options.idl
[modify] https://crrev.com/73b31c297c48b99f2d352625b74f159ef33e0f91/third_party/blink/renderer/modules/animationworklet/worklet_animation.cc