[css-transforms-2] Specify serialization of translate/rotate/scale better.

Author: tabatkins

css-transforms-2/Overview.bs

@@ -650,6 +650,58 @@ When 'translate', 'rotate' or 'scale' are animating or transitioning, and the fr
 value (but not both) is ''translate/none'', the value ''translate/none'' is replaced by the equivalent identity
 value (''0px'' for translate, ''0deg'' for rotate, ''1'' for scale).
 
+Serialization {#individual-transform-serialization}
+---------------------------------------------------
+
+Because these properties have three distinct modes of behavior
+(no transform, 2d transform, or 3d transform),
+serialization must take this into account:
+
+: for 'translate'
+:: If a 2d translation is specified,
+	the property must serialize with only one or two values
+	(per usual, if the second value is ''0px'', the default,
+	it must be omitted when serializing).
+
+	If a 3d translation is specified,
+	all three values must be serialized.
+
+	It must serialize as the keyword ''translate/none''
+	if and only if ''translate/none'' was originally specified.
+	(An identity transform does not count;
+	it must serialize as the 2d or 3d version,
+	as appropriate.)
+
+: for 'rotate'
+:: If a 2d rotation is specified,
+	the property must serialize as just an <<angle>>.
+
+	If a 3d rotation is specified,
+	the property must serialize with an axis specified.
+	If the axis is parallel with the x, y, or z axises,
+	it must serialize as the appropriate keyword.
+
+	It must serialize as the keyword ''rotate/none''
+	if and only if ''rotate/none'' was originally specified.
+	(An identity transform does not count;
+	it must serialize as the 2d or 3d version,
+	as appropriate.)
+
+: for 'scale'
+:: If a 2d scale is specified,
+	the property must serialize with only one or two values
+	(per usual, if the second value is the same as the first, the default,
+	it must be omitted when serializing).
+
+	If a 3d scale is specified,
+	all three values must be serialized.
+
+	It must serialize as the keyword ''scale/none''
+	if and only if ''scale/none'' was originally specified.
+	(An identity transform does not count;
+	it must serialize as the 2d or 3d version,
+	as appropriate.)
+
 Current Transformation Matrix {#ctm}
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