[css-values-4] Add generic <boolean> grammar, so the increasing number of places that use boolean exprs don't have to repeat themselves. w3c#10457

Author: tabatkins

css-values-4/Overview.bs

@@ -5345,6 +5345,101 @@ Appendix A: Recommended Minimum Ranges and Precision of Computed Values</h2>
 -->
 
 
+<!-- Big Text: bools -->
+
+<h2 id="bools">
+Boolean Expressions</h2>
+
+In several contexts, CSS allows certain boolean tests
+(things that can evaluate to true or false),
+and allows them to be combined with the logical operators
+AND, OR, and NOT to form more complicated boolean tests.
+(For example, ''@media'' or ''@supports'' rules.)
+
+All of these locations share a common grammatical structure,
+with the only difference being what "base" boolean tests
+are allowed,
+and whether they use simple 2-value (true/false) boolean algebra,
+or 3-value (true/false/unknown) Kleene logic.
+
+For example, in ''@media'' only <<media-condition>>s are allowed,
+with Kleene logic;
+in ''@supports'' only <<supports-condition>>s are allowed,
+with boolean logic.
+
+As the overall structure of these grammars is identical
+across the various usages of it,
+and also complex enough to be non-trivial to reproduce
+(and easy to accidentally do wrong),
+this specification defines <<boolean>>,
+a generic grammar for all such boolean tests.
+
+<pre class=prod>
+	<dfn>&lt;boolean></dfn> = <<bool-not>> | <<bool-in-parens>> [ <<bool-and>>* | <<bool-or>>* ]
+	<dfn>&lt;boolean-without-or></dfn> = <<bool-not>> | <<bool-in-parens>> <<bool-and>>*
+	<dfn>&lt;bool-not></dfn> = not <<bool-in-parens>>
+	<dfn>&lt;bool-and></dfn> = and <<bool-in-parens>>
+	<dfn>&lt;bool-or></dfn> = or <<bool-in-parens>>
+	<dfn>&lt;bool-in-parens></dfn> = ( <<boolean>> ) | <<bool-test>> | <<general-enclosed>>
+
+	<dfn>&lt;general-enclosed></dfn> = [ <<function-token>> <<any-value>>? ) ] | [ ( <<any-value>>? ) ]
+</pre>
+
+Each specification using <<boolean>>
+must define what things are valid as <dfn>&lt;bool-test></dfn> values
+in that context,
+and whether it uses 2-value or 3-value logic.
+As well, <<bool-test>> values
+<em>should</em> either be wrapped in parentheses
+or be functions;
+if a specification needs to express some conditions in a different way,
+please contact the CSSWG for review.
+
+: If using 2-value logic
+::
+	* Every <<bool-test>> must be defined to evaluate to either true or false.
+	* <<general-enclosed>> evaluates to false.
+	* <<bool-in-parens>> evaluates to the value of its contained <<boolean>>, <<bool-test>>, or <<general-enclosed>>
+	* <<bool-not>> evaluates to true if its contained <<bool-in-parens>> is false,
+		and false otherwise.
+	* A <<boolean>> with only a <<bool-not>> or <<bool-in-parens>> evaluates to that.
+	* A <<boolean>> with multiple <<bool-in-parens>> connected with ''and''
+		evaluates to true if all of its <<bool-in-parens>> are true,
+		and false otherwise.
+	* A <<boolean>> with multiple <<bool-in-parens>> connected with ''or''
+		evaluates to true if any of its <<bool-in-parens>> are true,
+		and false otherwise.
+
+: If using 3-value logic
+::
+	* Every <<bool-test>> must be defined to evaluate to either true, false, or unknown.
+	* <<general-enclosed>> evaluates to unknown.
+	* <<bool-in-parens>> evaluates to the value of its contained <<boolean>>, <<bool-test>>, or <<general-enclosed>>
+	* <<bool-not>> evaluates to true if its contained <<bool-in-parens>> is false,
+		false if it's true,
+		and unknown if it's unknown.
+	* A <<boolean>> with only a <<bool-not>> or <<bool-in-parens>> evaluates to that.
+	* A <<boolean>> with multiple <<bool-in-parens>> connected with ''and''
+		evaluates to true if all of its <<bool-in-parens>> are true,
+		false if any of them are false,
+		and unknown otherwise
+		(at least one unknown, but no false).
+	* A <<boolean>> with multiple <<bool-in-parens>> connected with ''or''
+		evaluates to true if any of its <<bool-in-parens>> are true,
+		false if all of them are false,
+		and unknown otherwise
+		(at least one unknown, but no true).
+	* If a "top-level" <<boolean>> is unknown,
+		and the containing context uses 2-value logic
+		(or doesn't specify otherwise),
+		it instead evaluates to false.
+
+		Note: That is, unknown doesn't "escape" a 3-value expression,
+		similar to how NaN doesn't "escape" a [=top-level calculation=]).
+
+
+
+
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