[css-values-4] Define the concept of calculation contexts more formally. #11204

Author: tabatkins

css-values-4/Overview.bs

@@ -4531,6 +4531,38 @@ Type Checking</h3>
 	might become valid in Quirks Mode
 	(and have its result interpreted as a ''px'' length).
 
+<h4 id='calc-context'>
+Calculation Contexts</h4>
+
+	Numeric values can be interpreted in various <dfn export>calculation contexts</dfn>,
+	depending on where they are used,
+	which defines how <<percentage>> values resolve,
+	etc.
+
+	<p class=example>
+		For example, in 'top', a <<percentage>> value
+		is resolved against the size of the containing block,
+		making it act as a <<length>>.
+		A single property can define multiple [=calculation contexts=];
+		for example, in 'background-size'
+		one value resolves <<percentages>> against the width of the [=background sizing area=]
+		while the other resolves them against the height.
+
+	[=Math functions=] always inherit the [=calculation context=] from wherever they're used.
+	That is, in ''top: calc(25% + 50px)'',
+	the <<percentage>> is resolved <em>as normal</em> for 'top',
+	as if ''top: 25%'' were specified
+	(and then has ''50px'' added to its value).
+
+	Unless otherwise defined,
+	all other functions <strong>do not inherit a [=calculation context=]</strong>,
+	and instead define their own [=calculation contexts=]
+	for their numeric arguments.
+	For example, in ''top: anchor(25%)'',
+	the <<percentage>> in the ''anchor()'' function
+	is instead defined to resolve in a completely different manner
+	(against the height of the default anchor element).
+
 <h4 id='calc-ieee'>
 Infinities, NaN, and Signed Zero</h4>