[css-values-5] Add definition of interpolate-size property.

This adds the 'interpolate-size' property as resolved in
w3c#10294

Fixes w3c#10294.

Author: dbaron

css-values-5/Overview.bs

@@ -2053,7 +2053,57 @@ between keywords and fixed lengths,
 even without an explicit ''calc-size()'' being used on one size?
 This would be nice, but compat analysis is required.
 
+<h3 id=interpolate-size>
+Interpolating sizing keywords: the 'interpolate-size' property</h3>
+
+Note: If we had a time machine, this property wouldn't need to exist.
+It exists because many existing style sheets assume that
+intrinsic sizing keywords
+(such as ''width/auto'', ''width/min-content'', etc.)
+cannot animate.
+Therefore this property exists to allow style sheets to choose
+to get the expected behavior.
+Specifying ''interpolate-size: allow-keywords'' on the root element
+chooses the new behavior for the entire page.
+We suggest doing this whenever compatibility isn't an issue.
+
+<pre class="propdef">
+Name: interpolate-size
+Value: ''numeric-only'' | ''allow-keywords''
+Initial: ''numeric-only''
+Inherited: yes
+Applies to: all elements
+Computed value: as specified
+Animation type: not animatable
+</pre>
 
+<dl dfn-for="interpolate-size" dfn-type="value">
+	<dt><dfn>numeric-only</dfn>
+	<dd>
+		An <<intrinsic-size-keyword>> cannot be interpolated.
+	</dd>
+
+	<dt><dfn>allow-keywords</dfn>
+	<dd>
+		Two values can be interpolated if
+		one of them is an <<intrinsic-size-keyword>>
+		and the other is a <<length-percentage>>.
+		This is done by treating
+		the <<intrinsic-size-keyword>> <var>keyword</var>
+		as though it is ''calc-size(<var>keyword</var>, size)''
+		and applying the rules in [[#interp-calc-size]].
+		In other cases,
+		an <<intrinsic-size-keyword>> still cannot be interpolated.
+	</dd>
+</dl>
+
+The value of 'interpolate-size' that matters
+is the computed value on the element
+at the time the animation might start.
+For CSS transitions,
+this means the value in the [=after-change style=].
+An animation is not stopped or started later
+because 'interpolate-size' changes.
 
 <!-- Big Text: etc
 
@@ -2115,4 +2165,4 @@ Security and Privacy Considerations</h2>
 	to be used in CSS values,
 	potentially exposing sensitive information
 	that was previously not accessible via CSS.
-	See [[#attr-security]].
+	See [[#attr-security]].