This might be a bit difficult to do as Blink currently treats something like 'var(--A), var(--b)' as a single CSSVariableReferenceValue, not two. To handle this properly, we'd probably have to do some tokenization ourselves to find all the vars. Furthermore, it looks like there are properties that can take both lists & single values (e.g. backdrop-filter), so not sure what should happen there.

For now, we just reject when we set with more than one variable reference (e.g. styleMap.set('transition-duration', '1s', 'var(--A)').

New resolution: We just reject when there's more than one variable reference value (same as current behaviour). We also need to reject when appending.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5014f339d1c7734cf21cde33780859515f460519

commit 5014f339d1c7734cf21cde33780859515f460519
Author: Darren Shen <shend@chromium.org>
Date: Mon Mar 05 21:17:35 2018

[css-typed-om] Fix crash when appending var refs.

We currently hit a DCHECK when appending a var ref. This patch fixes
the crash by rejecting it as per the spec.

Spec: https://github.com/w3c/css-houdini-drafts/commit/ecfcbf9721c0d7dff7890d8e009e4e3ad9b72d10

Bug:  814606 
Change-Id: I144307bf489db5e968c8e326087d13554ec3e59c
Reviewed-on: https://chromium-review.googlesource.com/948142
Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540938}
[modify] https://crrev.com/5014f339d1c7734cf21cde33780859515f460519/third_party/WebKit/LayoutTests/external/wpt/css/css-typed-om/the-stylepropertymap/declared/append.tentative.html
[modify] https://crrev.com/5014f339d1c7734cf21cde33780859515f460519/third_party/WebKit/LayoutTests/external/wpt/css/css-typed-om/the-stylepropertymap/declared/set.tentative.html
[modify] https://crrev.com/5014f339d1c7734cf21cde33780859515f460519/third_party/WebKit/LayoutTests/external/wpt/css/css-typed-om/the-stylepropertymap/inline/append.tentative.html
[modify] https://crrev.com/5014f339d1c7734cf21cde33780859515f460519/third_party/WebKit/LayoutTests/external/wpt/css/css-typed-om/the-stylepropertymap/inline/set.tentative.html
[modify] https://crrev.com/5014f339d1c7734cf21cde33780859515f460519/third_party/WebKit/Source/core/css/cssom/StylePropertyMap.cpp

Requesting merge for M66. Thanks!

Sorry, to be clear, the patch is the one in #c4.

Your change meets the bar and is auto-approved for M66. Please go ahead and merge the CL to branch 3359 manually. Please contact milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), josafat@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d4f22c85e7ae98d052793c328a88d8d19b344236

commit d4f22c85e7ae98d052793c328a88d8d19b344236
Author: Darren Shen <shend@chromium.org>
Date: Fri Mar 09 03:26:58 2018

[css-typed-om] Fix crash when appending var refs.

We currently hit a DCHECK when appending a var ref. This patch fixes
the crash by rejecting it as per the spec.

Spec: https://github.com/w3c/css-houdini-drafts/commit/ecfcbf9721c0d7dff7890d8e009e4e3ad9b72d10

Bug:  814606 
Change-Id: I144307bf489db5e968c8e326087d13554ec3e59c
Reviewed-on: https://chromium-review.googlesource.com/948142
Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#540938}(cherry picked from commit 5014f339d1c7734cf21cde33780859515f460519)
Reviewed-on: https://chromium-review.googlesource.com/956382
Cr-Commit-Position: refs/branch-heads/3359@{#130}
Cr-Branched-From: 66afc5e5d10127546cc4b98b9117aff588b5e66b-refs/heads/master@{#540276}
[modify] https://crrev.com/d4f22c85e7ae98d052793c328a88d8d19b344236/third_party/WebKit/LayoutTests/external/wpt/css/css-typed-om/the-stylepropertymap/declared/append.tentative.html
[modify] https://crrev.com/d4f22c85e7ae98d052793c328a88d8d19b344236/third_party/WebKit/LayoutTests/external/wpt/css/css-typed-om/the-stylepropertymap/declared/set.tentative.html
[modify] https://crrev.com/d4f22c85e7ae98d052793c328a88d8d19b344236/third_party/WebKit/LayoutTests/external/wpt/css/css-typed-om/the-stylepropertymap/inline/append.tentative.html
[modify] https://crrev.com/d4f22c85e7ae98d052793c328a88d8d19b344236/third_party/WebKit/LayoutTests/external/wpt/css/css-typed-om/the-stylepropertymap/inline/set.tentative.html
[modify] https://crrev.com/d4f22c85e7ae98d052793c328a88d8d19b344236/third_party/WebKit/Source/core/css/cssom/StylePropertyMap.cpp