[css-anchor-position] Define that env() flips its variable according to the try tactic. w3c#10422

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1554,11 +1554,20 @@ Values have the following meanings:
 
 	2. [=substitute a var()|Substitute variables=],
 		''env()'' functions,
-		and similar substitution functions
+		and similar [=arbitrary substitution functions=]
 		in |styles|.
 
-		Issue: Need to define a term to hook for substitution functions,
-		to make sure we can hit var(), env(), random(), etc...
+		For ''env()'' functions, 
+		if the referenced [=CSS/environment variable=] is associated with a direction or axis
+		(such as ''env()/safe-area-inset-top''),
+		switch the referenced [=CSS/environment variable=] corresponding to |directions|.
+
+		<div class=example>
+			For example, if ''top: env(safe-area-inset-top);'' is specified,
+			and |directions| are up and left,
+			the ''env()'' will resolve as if ''env(safe-area-inset-left)'' had been specified instead.
+			(And then, in the next step, will actually swap into the 'left' property.)
+		</div>
 
 	3. Swap the values of the |styles| between
 		the associated properties

css-env-1/Overview.bs

@@ -66,7 +66,7 @@ with a syntax like <code>values[0]</code>.
 Environment Variables {#environment}
 ====================================
 
-A CSS <dfn for=CSS>environment variable</dfn> is a name associated with a <<declaration-value>>
+A CSS <dfn export for=CSS>environment variable</dfn> is a name associated with a <<declaration-value>>
 (a sequence of zero more CSS tokens, with almost no restrictions on what tokens can exist),
 similar to a [=custom property=].
 [=Environment variables=] can be defined by the user agent,

css-variables-1/Overview.bs

@@ -858,7 +858,7 @@ Using Cascading Variables: the ''var()'' notation</h2>
 <h2 id=arbitrary-substitution>
 Arbitrary Substitution Functions</h2>
 
-	An <dfn>arbitrary substitution function</dfn>
+	An <dfn export>arbitrary substitution function</dfn>
 	is a function that will,
 	when resolved,
 	substitute itself with other content