It looks like we support:
  font-variant-ligatures
  font-variant-caps
  font-variant-numeric
From the spec, two more properties to go:
  font-variant-position
  font-variant-alternates
  font-variant-east-asian
https://drafts.csswg.org/css-fonts-3/#font-rend-props

and make them available in font-variant property and font-variant descriptor. Just parser work and plumbing I think.

drott@, is this in your radar?

>  font-variant-position

Requires additional logic for synthetisation.

>  font-variant-alternates

Requires a large set of additional CSS parsing logic to parse @font-feature-values at-rule and font-variant-alternates: bracketed expression syntax.

>  font-variant-east-asian

It looks as if this one is mostly about setting opentype feature flags, even for the ruby-value it seems as if this is only about activating the OpenType feature, not about any placement/layout changes. 

So the last one seems relatively straightforward. The other ones are definitely not. Yes, I am aware that there are properties that we have not implemented so far.

> it seems as if this is only about activating the OpenType feature, not about any placement/layout changes. 

Correct, it substitutes glyphs to the one designed for ruby.

Thanks drott! This bug is specifically about font-variant-east-asian. I'm glad my intuition matched yours here. It would be great if we could add the plumbing for this in Q2.

Is there any part of this which is not CSS plumbing? I'd like to remove the Blink>Fonts Component (CSS has a one component per bug policy to help with triaging and ownership).

There are parts of this where CSS values need to be used in FontBuilder to make a FontDescription, which then is used in HarfBuzzShaper to activate the respective OpenType features. This is relatively straightforward and follows the examples of font-variant-ligatures, etc. feel free to leave it in Blink>Fonts.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e0c82ed142b22229df697e782b690bf762135542

commit e0c82ed142b22229df697e782b690bf762135542
Author: Koji Ishii <kojii@chromium.org>
Date: Mon Aug 14 17:03:07 2017

Import wpt/css/css-fonts-3

This patch adds wpt/css/css-fonts-3 as a new import directory, as part
of supporting the 'font-variant-east-asian' property.

Following the new process since  crbug.com/713987 .

Bug:  712944 
Change-Id: I08641f311afe18cad4a95f4e1c582c7051b235ed
Reviewed-on: https://chromium-review.googlesource.com/611705
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494077}
[modify] https://crrev.com/e0c82ed142b22229df697e782b690bf762135542/third_party/WebKit/LayoutTests/W3CImportExpectations
[add] https://crrev.com/e0c82ed142b22229df697e782b690bf762135542/third_party/WebKit/LayoutTests/external/wpt/css/css-fonts-3/OWNERS

Let me merge to the launch tracking  issue 755462 .