[css-values-4] Define snapping negative lengths as line width

Author: fantasai

css-values-4/Overview.bs

@@ -1790,19 +1790,17 @@ Distance Units: the <<length>> type</h2>
 	(This algorithm is called by individual properties explicitly.)
 
 	<div algorithm>
-		To <dfn export lt="snap a length as a border width | snap as a border width">snap a length as a border width</dfn>
+		To <dfn export lt="snap as a line width | snap a length as a line width | snap a length as a border width | snap as a border width">snap a length as a line width</dfn>
 		given a <<length>> |len|:
 
-		1. Assert: |len| is non-negative.
-
-		2. If |len| is an integer number of [=device pixels=],
+		1. If |len| is an integer number of [=device pixels=],
 			do nothing.
 
-		3. If |len| is greater than zero, but less than 1 [=device pixel=],
-			round |len| up to 1 [=device pixel=].
+		2. If the absolute value of |len| is greater than zero, but less than 1 [=device pixel=],
+			round it away from zero to 1 [=device pixel=].
 
-		4. If |len| is greater than 1 [=device pixel=],
-			round it down to the nearest integer number of [=device pixels=].
+		4. If the absolute value of |len| is greater than 1 [=device pixel=],
+			round it towards zero to the nearest integer number of [=device pixels=].
 	</div>
 
 <h3 id="relative-lengths">
@@ -3344,18 +3342,13 @@ Stepped Value Functions: ''round()'', ''mod()'', and ''rem()''</h3>
 		:: 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=].
+		:: If B is omitted,
+			A is [=snapped as a line width=].
 
-			Otherwise, if A is non-negative and B is specified,
-			identical to ''<rounding-strategy>/nearest'',
+			Otherwise round as for ''<rounding-strategy>/nearest'',
 			except that if one of |lower B| or |upper B| is zero,
-			always choose the non-zero one.
-			Then [=snap as a border width=].
-
-			Otherwise (A is negative),
-			identical to ''<rounding-strategy>/nearest''.
-			(No snapping occurs.)
+			the non-zero one is chosen,
+			and the final result is [=snapped as a line width=].
 	</dl>
 
 	If |lower B| would be zero,
@@ -5671,6 +5664,12 @@ Recent Changes</h3>
 
 	(This is a subset of [[#additions-L3]].)
 
+	Substantial changes since <a href="https://www.w3.org/TR/2024/WD-css-values-4-20240312/">12 March 2024 Working Draft</a>:
+	* Adapt the [=snap as a line width=] algorithm to handle negative numbers.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/13795">Issue 13795</a>)
+
+	ISSUE: Finish this list.
+
 	Substantial changes since <a href="https://www.w3.org/TR/2023/WD-css-values-4-20231218/">18 December 2023 WD</a>:
 
 	* Added the ''clamp()/none'' values to ''clamp()'',
@@ -5719,7 +5718,7 @@ Recent Changes</h3>
 	<ul>
 		<li>Added [[#component-functions]] to formally define the way that [=functional notation=] syntaxes are defined.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/2921">Issue 2921</a>)
-		<li>Added algorithm for [=snap as a border width=],
+		<li>Added algorithm for [=snap as a line width=],
 			to reflect the interoperable rules for rendering consistent stroke widths.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/5210">Issue 5210</a>)
 		<li>Clarified grammar and [=computed value=] of ''mix()''.