[css-values-4][various] Define 'snapped as a border width', and invoke it in several properties to snap computed values. #5210

Author: tabatkins

css-backgrounds-3/Overview.bs

@@ -1382,7 +1382,7 @@ Initial: medium
 Applies to: all elements except [=ruby base containers=] and [=ruby annotation containers=]
 Inherited: no
 Percentages: N/A
-Computed value: absolute length; zero if the border style is ''border-style/none'' or ''hidden''
+Computed value: absolute length, [=snapped as a border width=]; zero if the border style is ''border-style/none'' or ''hidden''
 Animatable: by computed value
 </pre>
 

css-multicol-1/Overview.bs

@@ -872,7 +872,7 @@ Stacking Context</h3>
 	Applies to: multicol containers
 	Inherited: no
 	Percentages: N/A
-	Computed value: absolute length; ''0'' if the column rule style is ''border-style/none'' or ''hidden''
+	Computed value: absolute length, [=snapped as a border width=]; ''0'' if the column rule style is ''border-style/none'' or ''hidden''
 	Animation type: by computed value type
 	</pre>
 

css-ui-3/Overview.bs

@@ -322,7 +322,7 @@ Initial: medium
 Applies to: all elements
 Inherited: no
 Percentages: N/A
-Computed value: absolute length; ''0'' if the outline style is ''border-style/none''.
+Computed value: absolute length, [=snapped as a border width=]; ''0'' if the outline style is ''border-style/none''.
 Animation type: by computed value
 </pre>
 

css-ui-4/Overview.bs

@@ -264,7 +264,7 @@ Initial: medium
 Applies to: all elements
 Inherited: no
 Percentages: N/A
-Computed value: absolute length; ''0'' if the outline style is ''border-style/none''.
+Computed value: absolute length, [=snapped as a border width=]; ''0'' if the outline style is ''border-style/none''.
 Animation type: by computed value
 </pre>
 

css-values-4/Overview.bs

@@ -670,7 +670,7 @@ Unprefixed Author-defined Identifiers: the <<custom-ident>> type</h3>
 	(e.g. ''example'' and ''EXAMPLE'' are two different, unrelated user-defined identifiers).
 
 	The <a>CSS-wide keywords</a> are not valid <<custom-ident>>s.
-	The ''default'' keyword is reserved
+	The <css>default</css> keyword is reserved
 	and is also not a valid <<custom-ident>>.
 	Specifications using <<custom-ident>> must specify clearly
 	what other keywords are excluded from <<custom-ident>>, if any--
@@ -1661,6 +1661,30 @@ Distance Units: the <<length>> type</h2>
 	css/css-shapes/shape-outside/values/shape-outside-polygon-004.html
 	</wpt>
 
+	While the exact supported precision of numeric values,
+	and how they are rounded to match that precision,
+	is generally intentionally undefined,
+	<<length>>s in 'border-width' and a few other properties
+	are rounded in a specific fashion
+	to ensure reasonable visual display.
+	(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>
+		given a <<length>> |len|:
+
+		1. Assert: |len| is non-negative.
+
+		2. 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=].
+
+		4. If |len| is greater than 1 [=device pixel=],
+			round it down to the nearest integer number of [=device pixels=].
+	</div>
+
 <h3 id="relative-lengths">
 Relative Lengths</h3>
 
@@ -1827,7 +1851,7 @@ Font-relative Lengths: the ''em'', ''rem'', ''ex'', ''rex'', ''cap'', ''rcap'',
 		<dd>
 			Equal to the value of the ''cap'' unit on the root element.
 
-		<dt><dfn id="ch" caniuse="ch">ch</dfn>
+		<dt><dfn id="ch" caniuse="ch-unit">ch</dfn>
 		<dd>
 			Represents the typical <a>advance measure</a> of European alphanumeric characters,
 			and measured as the used <a>advance measure</a> of the “0” (ZERO, U+0030) glyph