It's part of CSS Cascading and Inheritance Level 3:
See also https://developer.mozilla.org/en-US/docs/Web/CSS/unset
Chrome and Firefox already support it, and it's "In Development" for Microsoft Edge.
display: unset;/* in this case, same as inline */
Created attachment 263197 [details]
Attachment 263197 [details] did not pass style-queue:
ERROR: Source/WebCore/css/CSSValue.h:49: enum members should use InterCaps with an initial capital letter. [readability/enum_casing] 
ERROR: Source/WebCore/css/CSSValue.h:50: enum members should use InterCaps with an initial capital letter. [readability/enum_casing] 
Total errors found: 2 in 23 files
If any of these errors are false positives, please file a bug against check-webkit-style.
Comment on attachment 263197 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=263197&action=review
> + * Copyright (C) 2015 Apple Inc. All rights reserved.
> + *
> + * This library is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Library General Public
> + * License as published by the Free Software Foundation; either
> + * version 2 of the License, or (at your option) any later version.
> + *
> + * This library is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + * Library General Public License for more details.
> + *
> + * You should have received a copy of the GNU Library General Public License
> + * along with this library; see the file COPYING.LIB. If not, write to
> + * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
> + * Boston, MA 02110-1301, USA.
I thought we used this one for new files:
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Or maybe it doesn't matter.
Fixed in r191151.