[mediaqueries] Define 3-value logic for media queries.

Author: tabatkins

mediaqueries/Overview.bs

@@ -170,11 +170,9 @@ Media Queries</h2>
 		matches the media type of the device where the user agent is running, and
 	* the <a>media condition</a> is true.
 
-	Statements regarding media queries in this section assume the
-	<a href="#mq-syntax">syntax section</a> is followed. Media queries that do not
-	conform to the syntax are discussed in the
-	<a href="#error-handling">error handling section</a>. I.e. the syntax takes
-	precedence over requirements in this section.
+	Statements regarding media queries in this section assume the <a href="#mq-syntax">syntax section</a> is followed.
+	Media queries that do not conform to the syntax are discussed in [[#error-handling]].
+	I.e. the syntax takes precedence over requirements in this section.
 
 	<div class="example">
 		Here is a simple example written in HTML:
@@ -714,7 +712,7 @@ Syntax</h2>
 	<dfn>&lt;mf-name></dfn> = <<ident>>
 	<dfn>&lt;mf-value></dfn> = <<number>> | <<dimension>> | <<ident>> | <<ratio>>
 
-	<dfn>&lt;general-enclosed></dfn> = [ <<function-token>> | ( ] <<any-value>>* )
+	<dfn>&lt;general-enclosed></dfn> = [ <<function-token>> <<any-value>> ) ] | ( <<ident>> <<any-value>> )
 	</pre>
 
 	The <<media-type>> production does not include the keywords ''only'', ''not'', ''and'', and ''or''.
@@ -749,29 +747,30 @@ Syntax</h2>
 	<dl>
 		<dt><<media-condition>>
 		<dt><<media-condition-without-or>>
+		<dt><<media-in-parens>>
 		<dd>
 			The result is the result of the child term.
 
 		<dt><<media-not>>
 		<dd>
 			The result is the negation of the <<media-in-parens>> term.
+			The negation of unknown is unknown.
 
 		<dt><<media-and>>
 		<dd>
 			The result is true if all of the <<media-in-parens>> child terms are true,
-			and false otherwise.
+			false if at least one of the <<media-in-parens>> child terms are false,
+			and unknown otherwise.
 
 		<dt><<media-or>>
 		<dd>
-			The result is true if any of the <<media-in-parens>> child terms are true,
-			and false otherwise.
+			The result is false if all of the <<media-in-parens>> child terms are false,
+			true if at least one of the <<media-in-parens>> child terms are true,
+			and unknown otherwise.
 
-		<dt><<media-in-parens>>
+		<dt><<general-enclosed>>
 		<dd>
-			If it contains a <<media-in-parens>> or <<media-feature>> child term,
-			the result is the result of that child term.
-			If it contains a <<general-enclosed>> child term,
-			the result is false.
+			The result is unknown.
 
 			Authors must not use <<general-enclosed>> in their stylesheets.
 			<span class='note'>It exists only for future-compatibility,
@@ -782,6 +781,30 @@ Syntax</h2>
 			The result is the result of evaluating the specified media feature.
 	</dl>
 
+	If the result of a <a>media query</a> is unknown,
+	it must be treated as false for the purpose of matching.
+
+	<div class="note">
+		Media Queries use a three-value logic where terms can be "true", "false", or "unknown".
+		Specifically, it uses the <a href="http://en.wikipedia.org/wiki/Three-valued_logic#Kleene_and_Priest_logics">Kleene 3-valued logic</a>.
+		In this logic, "unknown" means "either true or false, but we're not sure which yet".
+
+		In general, an unknown value showing up in a formula will cause the formula to be unknown as well,
+		as substituting "true" for the unknown will give the formula a different result than substituting "false".
+		The only way to eliminate an unknown value is to use it in a formula that will give the same result
+		whether the unknown is replaced with a true or false value.
+		This occurs when you have "false AND unknown" (evaluates to false regardless)
+		and "true OR unknown" (evaluates to true regardless).
+
+		This logic was adopted because <<general-enclosed>> needs to be assigned a truth value.
+		In standard boolean logic, the only reasonable value is "false",
+		but this means that ''not unknown(function)'' is true,
+		which can be confusing an unwanted.
+		Kleen's 3-valued logic ensures that unknown things will prevent a <a>media query</a> from matching,
+		unless their value is irrelevant to the final result.
+	</div>
+
+
 <h3 id="error-handling">
 Error Handling</h3>