diff --git a/testing/web-platform/tests/css/css-backgrounds/inheritance.sub.html b/testing/web-platform/tests/css/css-backgrounds/inheritance.sub.html
index efd6615f4c8dd..ee029f24f41a9 100644
--- a/testing/web-platform/tests/css/css-backgrounds/inheritance.sub.html
+++ b/testing/web-platform/tests/css/css-backgrounds/inheritance.sub.html
@@ -66,6 +66,38 @@
 "
 >
 <
+link
+rel
+=
+"
+help
+"
+href
+=
+"
+https
+:
+/
+/
+drafts
+.
+csswg
+.
+org
+/
+css
+-
+backgrounds
+-
+4
+/
+#
+property
+-
+index
+"
+>
+<
 meta
 name
 =
@@ -534,6 +566,42 @@
 '
 background
 -
+position
+-
+x
+'
+'
+0
+%
+'
+'
+10px
+'
+)
+;
+assert_not_inherited
+(
+'
+background
+-
+position
+-
+y
+'
+'
+0
+%
+'
+'
+20px
+'
+)
+;
+assert_not_inherited
+(
+'
+background
+-
 repeat
 '
 '
diff --git a/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-x-computed.html b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-x-computed.html
new file mode 100644
index 0000000000000..a02d75411e254
--- /dev/null
+++ b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-x-computed.html
@@ -0,0 +1,501 @@
+<
+!
+DOCTYPE
+html
+>
+<
+html
+>
+<
+head
+>
+<
+meta
+charset
+=
+"
+utf
+-
+8
+"
+>
+<
+title
+>
+CSS
+Backgrounds
+and
+Borders
+Module
+Level
+4
+:
+getComputedValue
+(
+)
+.
+backgroundPositionX
+<
+/
+title
+>
+<
+link
+rel
+=
+"
+help
+"
+href
+=
+"
+https
+:
+/
+/
+drafts
+.
+csswg
+.
+org
+/
+css
+-
+backgrounds
+-
+4
+/
+#
+propdef
+-
+background
+-
+position
+-
+x
+"
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharness
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharnessreport
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+css
+/
+support
+/
+computed
+-
+testcommon
+.
+js
+"
+>
+<
+/
+script
+>
+<
+style
+>
+#
+target
+{
+font
+-
+size
+:
+40px
+;
+}
+<
+/
+style
+>
+<
+/
+head
+>
+<
+body
+>
+<
+div
+id
+=
+"
+target
+"
+>
+<
+/
+div
+>
+<
+script
+>
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+center
+"
+"
+50
+%
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+left
+"
+"
+0
+%
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+right
+"
+"
+100
+%
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+x
+-
+start
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+x
+-
+end
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+-
+20
+%
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+10px
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+0
+.
+5em
+"
+"
+20px
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+calc
+(
+10px
+-
+0
+.
+5em
+)
+"
+"
+-
+10px
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+left
+-
+20
+%
+"
+"
+-
+20
+%
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+right
+10px
+"
+"
+calc
+(
+100
+%
++
+10px
+)
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+-
+20
+%
+10px
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+center
+left
+right
+"
+"
+50
+%
+0
+%
+100px
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+0
+.
+5em
+x
+-
+start
+x
+-
+end
+"
+"
+20px
+x
+-
+start
+x
+-
+end
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+calc
+(
+10px
+-
+0
+.
+5em
+)
+left
+-
+20
+%
+right
+10px
+"
+"
+-
+10px
+-
+20
+%
+calc
+(
+100
+%
++
+10px
+)
+"
+)
+;
+<
+/
+script
+>
+<
+/
+body
+>
+<
+/
+html
+>
diff --git a/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-x-invalid.html b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-x-invalid.html
new file mode 100644
index 0000000000000..2beb105a4c633
--- /dev/null
+++ b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-x-invalid.html
@@ -0,0 +1,346 @@
+<
+!
+DOCTYPE
+html
+>
+<
+html
+>
+<
+head
+>
+<
+meta
+charset
+=
+"
+utf
+-
+8
+"
+>
+<
+title
+>
+CSS
+Backgrounds
+and
+Borders
+Module
+Level
+4
+:
+parsing
+background
+-
+position
+-
+x
+with
+invalid
+values
+<
+/
+title
+>
+<
+link
+rel
+=
+"
+help
+"
+href
+=
+"
+https
+:
+/
+/
+drafts
+.
+csswg
+.
+org
+/
+css
+-
+backgrounds
+-
+4
+/
+#
+propdef
+-
+background
+-
+position
+-
+x
+"
+>
+<
+meta
+name
+=
+"
+assert
+"
+content
+=
+"
+background
+-
+position
+-
+x
+supports
+only
+the
+grammar
+'
+[
+center
+|
+[
+left
+|
+right
+|
+x
+-
+start
+|
+x
+-
+end
+]
+?
+<
+length
+-
+percentage
+>
+?
+]
+#
+'
+.
+"
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharness
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharnessreport
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+css
+/
+support
+/
+parsing
+-
+testcommon
+.
+js
+"
+>
+<
+/
+script
+>
+<
+/
+head
+>
+<
+body
+>
+<
+script
+>
+test_invalid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+top
+"
+)
+;
+test_invalid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+bottom
+"
+)
+;
+test_invalid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+y
+-
+start
+"
+)
+;
+test_invalid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+y
+-
+end
+"
+)
+;
+test_invalid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+center
+10px
+"
+)
+;
+test_invalid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+20
+%
+left
+"
+)
+;
+test_invalid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+right
+left
+"
+)
+;
+test_invalid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+x
+-
+start
+center
+"
+)
+;
+test_invalid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+left
+center
+right
+"
+)
+;
+<
+/
+script
+>
+<
+/
+body
+>
+<
+/
+html
+>
diff --git a/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-x-valid.html b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-x-valid.html
new file mode 100644
index 0000000000000..910a88a0cce99
--- /dev/null
+++ b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-x-valid.html
@@ -0,0 +1,462 @@
+<
+!
+DOCTYPE
+html
+>
+<
+html
+>
+<
+head
+>
+<
+meta
+charset
+=
+"
+utf
+-
+8
+"
+>
+<
+title
+>
+CSS
+Backgrounds
+and
+Borders
+Module
+Level
+4
+:
+parsing
+background
+-
+position
+-
+x
+with
+valid
+values
+<
+/
+title
+>
+<
+link
+rel
+=
+"
+help
+"
+href
+=
+"
+https
+:
+/
+/
+drafts
+.
+csswg
+.
+org
+/
+css
+-
+backgrounds
+-
+4
+/
+#
+propdef
+-
+background
+-
+position
+-
+x
+"
+>
+<
+meta
+name
+=
+"
+assert
+"
+content
+=
+"
+background
+-
+position
+-
+x
+supports
+the
+full
+grammar
+'
+[
+center
+|
+[
+left
+|
+right
+|
+x
+-
+start
+|
+x
+-
+end
+]
+?
+<
+length
+-
+percentage
+>
+?
+]
+#
+'
+.
+"
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharness
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharnessreport
+.
+js
+"
+>
+<
+/
+script
+>
+<
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+=
+"
+/
+css
+/
+support
+/
+parsing
+-
+testcommon
+.
+js
+"
+>
+<
+/
+script
+>
+<
+/
+head
+>
+<
+body
+>
+<
+script
+>
+test_valid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+center
+"
+)
+;
+test_valid_value
+(
+"
+background
+-
+position
+-
+x
+"
+"
+left
+"
+)
+;
+test_valid_value
+(
+"
+background
+-
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+-
+x
+"
+"
+right
+"
+)
+;
+test_valid_value
+(
+"
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+-
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+-
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+"
+"
+x
+-
+start
+"
+)
+;
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+(
+"
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+-
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+-
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+"
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+-
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+)
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+"
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+-
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+-
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+"
+-
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+"
+)
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+(
+"
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+-
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+-
+x
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+"
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+"
+)
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+(
+"
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+-
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+-
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+"
+"
+0
+.
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+"
+)
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+(
+"
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+-
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+-
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+"
+"
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+(
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+-
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+.
+5em
+)
+"
+)
+;
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+(
+"
+background
+-
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+-
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+"
+"
+left
+-
+20
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+"
+)
+;
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+(
+"
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+-
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+-
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+"
+"
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+"
+)
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+"
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+-
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+-
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+"
+-
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+)
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+-
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+"
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+"
+)
+;
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+"
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+-
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+-
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+"
+"
+0
+.
+5em
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+-
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+-
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+"
+)
+;
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+"
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+-
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+-
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+"
+calc
+(
+10px
+-
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+)
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+-
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+"
+)
+;
+<
+/
+script
+>
+<
+/
+body
+>
+<
+/
+html
+>
diff --git a/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-y-computed.html b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-y-computed.html
new file mode 100644
index 0000000000000..f07f141b7eef5
--- /dev/null
+++ b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-y-computed.html
@@ -0,0 +1,501 @@
+<
+!
+DOCTYPE
+html
+>
+<
+html
+>
+<
+head
+>
+<
+meta
+charset
+=
+"
+utf
+-
+8
+"
+>
+<
+title
+>
+CSS
+Backgrounds
+and
+Borders
+Module
+Level
+4
+:
+getComputedValue
+(
+)
+.
+backgroundPositionY
+<
+/
+title
+>
+<
+link
+rel
+=
+"
+help
+"
+href
+=
+"
+https
+:
+/
+/
+drafts
+.
+csswg
+.
+org
+/
+css
+-
+backgrounds
+-
+4
+/
+#
+propdef
+-
+background
+-
+position
+-
+y
+"
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharness
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+resources
+/
+testharnessreport
+.
+js
+"
+>
+<
+/
+script
+>
+<
+script
+src
+=
+"
+/
+css
+/
+support
+/
+computed
+-
+testcommon
+.
+js
+"
+>
+<
+/
+script
+>
+<
+style
+>
+#
+target
+{
+font
+-
+size
+:
+40px
+;
+}
+<
+/
+style
+>
+<
+/
+head
+>
+<
+body
+>
+<
+div
+id
+=
+"
+target
+"
+>
+<
+/
+div
+>
+<
+script
+>
+test_computed_value
+(
+"
+background
+-
+position
+-
+y
+"
+"
+center
+"
+"
+50
+%
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+y
+"
+"
+top
+"
+"
+0
+%
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+y
+"
+"
+bottom
+"
+"
+100
+%
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+y
+"
+"
+y
+-
+start
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+y
+"
+"
+y
+-
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+"
+)
+;
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+(
+"
+background
+-
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+-
+y
+"
+"
+-
+20
+%
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+y
+"
+"
+10px
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+y
+"
+"
+0
+.
+5em
+"
+"
+20px
+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+y
+"
+"
+calc
+(
+10px
+-
+0
+.
+5em
+)
+"
+"
+-
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+"
+)
+;
+test_computed_value
+(
+"
+background
+-
+position
+-
+y
+"
+"
+top
+-
+20
+%
+"
+"
+-
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+%
+"
+)
+;
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+(
+"
+background
+-
+position
+-
+y
+"
+"
+bottom
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+"
+"
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+(
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+%
++
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+)
+"
+)
+;
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+(
+"
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+-
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+-
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+"
+"
+-
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+"
+)
+;
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+"
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+-
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+-
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+"
+"
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+"
+"
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+%
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+"
+)
+;
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+(
+"
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+-
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+-
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+"
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+.
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+-
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+-
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+"
+"
+20px
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+-
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+-
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+"
+)
+;
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+(
+"
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+-
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+-
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+"
+"
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+(
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+-
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+.
+5em
+)
+top
+-
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+%
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+"
+"
+-
+10px
+-
+20
+%
+calc
+(
+100
+%
++
+10px
+)
+"
+)
+;
+<
+/
+script
+>
+<
+/
+body
+>
+<
+/
+html
+>
diff --git a/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-y-invalid.html b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-y-invalid.html
new file mode 100644
index 0000000000000..59c1120e64be8
--- /dev/null
+++ b/testing/web-platform/tests/css/css-backgrounds/parsing/background-position-y-invalid.html
@@ -0,0 +1,346 @@
+<
+!
+DOCTYPE
+html
+>
+<
+html
+>
+<
+head
+>
+<
+meta
+charset
+=
+"
+utf
+-
+8
+"
+>
+<
+title
+>
+CSS
+Backgrounds
+and
+Borders
+Module
+Level
+4
+:
+parsing
+background
+-
+position
+-
+y
+with
+invalid
+values
+<
+/
+title
+>
+<
+link
+rel
+=
+"
+help
+"
+href
+=
+"
+https
+:
+/
+/
+drafts
+.
+csswg
+.
+org
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