[css-values-4] Add abs() and sign().

Author: tabatkins

css-values-4/Overview.bs

@@ -3059,6 +3059,41 @@ Argument Ranges</h4>
 	</div>
 
 
+<h3 id='calc-signs'>
+Sign-Related Functions: ''abs()'', ''sign()''</h3>
+
+	The sign-related functions--
+	''abs()'' and ''sign()''--
+	compute various functions related to the sign of their argument.
+
+	The <dfn lt="abs()">abs(A)</dfn> function
+	contains one [=calculation=] A,
+	and returns the absolute value of A
+	(if A is non-negative, just A;
+	if A is negative, ''-1 * A'')
+	as the same [=CSSNumericValue/type=] as the input.
+
+	The <dfn lt="sign()">sign(A)</dfn> function
+	contains one [=calculation=] A,
+	and returns -1 if A is negative,
+	+1 if A is positive,
+	0⁺ if A is 0⁺,
+	and 0⁻ if A is 0⁻.
+
+	Note: Both of these functions operate on the fully simplified/resolved form of their arguments,
+	which may give unintuitive results at first glance.
+	In particular, an expression like ''10%''
+	an expression like ''10%'' might be positive <em>or</em> negative once it's resolved,
+	depending on what value it's resolved against.
+	For example, in 'background-position' positive percentages
+	resolve to a negative length,
+	and vice versa,
+	if the background image is larger than the background area.
+	Thus ''sign(10%)'' might return ''1'' <em>or</em> ''-1'',
+	depending on how the percentage is resolved!
+	(Or even ''0'', if it's resolved against a zero length.)
+
+
 <h3 id='calc-syntax'>
 Syntax</h3>
 
@@ -3081,6 +3116,8 @@ Syntax</h3>
 	<<hypot()>> = hypot( <<calc-sum>># )
 	<<log()>>   = log( <<calc-sum>>, <<calc-sum>>? )
 	<<exp()>>   = exp( <<calc-sum>> )
+	<<abs()>>   = abs( <<calc-sum>> )
+	<<sign()>>  = sign( <<calc-sum>> )
 	<dfn>&lt;calc-sum></dfn> = <<calc-product>> [ [ '+' | '-' ] <<calc-product>> ]*
 	<dfn>&lt;calc-product></dfn> = <<calc-value>> [ [ '*' | '/' ] <<calc-value>> ]*
 	<dfn>&lt;calc-value></dfn> = <<number>> | <<dimension>> | <<percentage>> | ( <<calc-sum>> )