Ignoring right and bottom, the following are permitted by the <bg-position> but not <position>
> center left <length-percentage>
> center top <length-percentage>
> left <length-percentage> center
> left <length-percentage> top
> left top <length-percentage>
> top left <length-percentage>
> top <length-percentage> center
> top <length-percentage> left

The following are permitted by <position> and begin with cases above:
> left <length-percentage> top <length-percentage>
> top <length-percentage> left <length-percentage>

The syntax of position in CSS Values and Units 3 has recently changed
https://bugs.chromium.org/p/chromium/issues/detail?id=717833
https://drafts.csswg.org/css-values-3/#position

Should scroll-snap-coordinate and scroll-snap-destination be changed to use the new syntax?

Or should they be left unchanged?

Or should we delete the implementation of scroll-snap-coordinate and scroll-snap-destination as they don't appear in specs published after
https://www.w3.org/TR/2015/WD-css-snappoints-1-20150326/#scroll-snap-coordinate
 
I suggest using the new syntax for the unshipped offset-position and offset-anchor, and add one or more use counters for
-object-position
-perspective-origin
-center position for circle and ellipse basic shapes
-radial gradient
-conic gradient
in preparation for an Intent to Deprecate and Remove support for three valued positions.

background-position would continue to support three valued positions.

For scroll-snap-coordinate and scroll-snap-destination. We are going to remove those in favor of implementing the new syntax. But that happens when the patch for the new syntax lands. Here is the WIP: https://codereview.chromium.org/2767213003/

Use counter CL uploaded https://codereview.chromium.org/2888283006/

Should unshipped conic gradient use the new position grammar that forbids three values? Or use the same legacy position grammar as shipped radial gradient?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a951d7b5e8647b077be37f420254b73063a30b4c

commit a951d7b5e8647b077be37f420254b73063a30b4c
Author: ericwilligers <ericwilligers@chromium.org>
Date: Wed May 24 18:21:31 2017

CSS: Use count position values with 3 parts

The CSS Working Group recently changed the grammar of <position> such
that 3-value syntax is no longer supported.
https://lists.w3.org/Archives/Public/www-style/2017Mar/0054.html

We update parsing logic so that the following now use count three value
positions, in preparation for future deprecation and removal discussions:
object-position https://drafts.csswg.org/css-images-3/#the-object-position
perspective-origin
https://drafts.csswg.org/css-transforms-2/#perspective-origin-property
scroll-snap-coordinate (not shipped, not in current spec)
https://www.w3.org/TR/2015/WD-css-snappoints-1-20150326/#scroll-snap-coordinate
scroll-snap-destination (not shipped, not in current spec)
https://www.w3.org/TR/2015/WD-css-snappoints-1-20150326/#scroll-snap-destination
center position for circle and ellipse basic shapes
https://drafts.csswg.org/css-shapes/#supported-basic-shapes
(used in https://www.w3.org/TR/css-shapes-1/#shape-outside-property and
https://drafts.fxtf.org/css-masking-1/#the-clip-path and
https://drafts.fxtf.org/motion-1/#offset-path-property )
position for radial and (not shipped) conic gradients
https://drafts.csswg.org/css-images-3/#radial-gradients
https://drafts.csswg.org/css-images-4/#conic-gradients
(used in https://drafts.csswg.org/css-backgrounds-3/#the-background-image )

The following no longer accept three value positions:
offset-anchor (property not yet shipped, expecting to ship soon)
https://drafts.fxtf.org/motion-1/#offset-anchor-property
offset-position (property not yet shipped, expecting to ship soon)
https://drafts.fxtf.org/motion-1/#offset-position-property

The following are not affected:
background-position
https://drafts.csswg.org/css-backgrounds-3/#background-position
(continues to support three values)
transform-origin
https://drafts.csswg.org/css-transforms/#transform-origin-property
(has never used <position>, instead defines its own grammar)

BUG= 717833 

Review-Url: https://codereview.chromium.org/2888283006
Cr-Commit-Position: refs/heads/master@{#474358}

[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/css3/motion-path/offset-position.html
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/scroll-snap-parsing-expected.txt
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/scroll-snap-parsing.html
[add] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/usecounter-position3value-background-position.html
[add] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/usecounter-position3value-background.html
[add] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/usecounter-position3value-basic-shape-circle.html
[add] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/usecounter-position3value-basic-shape-ellipse.html
[add] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/usecounter-position3value-conic-gradient.html
[add] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/usecounter-position3value-object-position.html
[add] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/usecounter-position3value-perspective-origin.html
[add] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/LayoutTests/fast/css/usecounter-position3value-radial-gradient.html
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/Source/core/css/parser/CSSPropertyParser.cpp
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/Source/core/css/parser/CSSPropertyParserHelpers.cpp
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/Source/core/css/parser/CSSPropertyParserHelpers.h
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/Source/core/css/properties/CSSPropertyAPIOffsetAnchor.cpp
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/Source/core/css/properties/CSSPropertyAPIOffsetPosition.cpp
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/Source/core/css/properties/CSSPropertyAPIScrollSnapCoordinate.cpp
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/Source/core/css/properties/CSSPropertyShapeUtils.cpp
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/a951d7b5e8647b077be37f420254b73063a30b4c/tools/metrics/histograms/enums.xml