[css-values-4] Add round(line-width, ...). #3720

Author: tabatkins

css-values-4/Overview.bs

@@ -3323,7 +3323,7 @@ Stepped Value Functions: ''round()'', ''mod()'', and ''rem()''</h3>
 
 	If A is exactly equal to an integer multiple of B,
 	''round()'' resolves to A exactly
-	(preserving whether A is 0⁻	or 0⁺, if relevant).
+	(preserving whether A is 0⁻ or 0⁺, if relevant).
 	Otherwise, there are two integer multiples of B that are potentially "closest" to A,
 	|lower B| which is closer to −∞
 	and |upper B| which is closer to +∞.
@@ -3343,6 +3343,15 @@ Stepped Value Functions: ''round()'', ''mod()'', and ''rem()''</h3>
 		: <dfn>to-zero</dfn>
 		:: Choose whichever of |lower B| and |upper B|
 			that has the smallest absolute difference from 0.
+		: <dfn>line-width</dfn>
+		:: If A is non-negative and B is omitted,
+			A is [=snapped as a border width=].
+
+			Otherwise, identical to ''<rounding-strategy>/nearest'',
+			except that if A is positive (non-zero)
+			and one of |lower B| or |upper B| is zero,
+			choose the non-zero one,
+			then [=snap as a border width=].
 	</dl>
 
 	If |lower B| would be zero,
@@ -3353,11 +3362,28 @@ Stepped Value Functions: ''round()'', ''mod()'', and ''rem()''</h3>
 	If <<rounding-strategy>> is omitted,
 	it defaults to ''nearest''.
 	(Aka [=rounding to the nearest integer=].)
+	If <<rounding-strategy>> is ''<rounding-strategy>/line-width'',
+	the [=CSSNumericValue/type=] of A
+	must match <<length>>.
+
 	If the [=CSSNumericValue/type=] of A
 	matches <<number>>,
 	then B may be omitted,
-	and defaults to ''1'';
-	omitting B is otherwise invalid.
+	and defaults to ''1''.
+	If <<rounding-strategy>> is ''<rounding-strategy>/line-width'',
+	B may also be omitted
+	and has special behavior (defined above)
+	essentially defaulting it to one device pixel.
+	In all other cases,
+	omitting B is invalid.
+
+	Advisement: While ''round(line-width, ...)''
+	attempts to ensure that non-zero values never round to zero,
+	passing a sufficiently small A value
+	might cause it to be represented as 0 internally anyway.
+	Use caution when writing very small values;
+	''round(line-width, 0.1px)'' is safer than ''round(line-width, 0.0001px)'',
+	and the difference between the two is almost certainly nil in practice.
 
 	Issue(5689): CSSOM needs to specify how it rounds,
 	and it's probably good for CSS functions to round the same way by default.