[selectors-4][editorial] Clarify the expected rules for :blank, matching what HTML is expected to specify. w3c#8220

Author: tabatkins

selectors-4/Overview.bs

@@ -2743,15 +2743,25 @@ The Empty-Value Pseudo-class: '':blank''</h4>
 	applies to user-input elements whose input value is empty
 	(consists of the empty string or otherwise null input).
 
-	<div class="example">
-		Examples of '':blank'' user-input elements would be
-		a <{textarea}> element whose contents are empty,
-		or an <{input}> field whose value is empty.
-		Note that the value under consideration here is the value
-		that would be submitted
-		(see <a href="https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#a-form-control's-value">A form control’s value</a> in [[HTML]]),
-		which in HTML does not necessarily correspond to the value
-		of the element’s <{input/value}> attribute.
+	<div class="note">
+		Roughly speaking, if a human looked at a printout of the form
+		and would say it’s blank,
+		it matches '':blank''.
+
+		A rule of thumb for interpreting '':blank'' on form controls is:
+
+		* If the control always submits,
+			and would do so with an empty string, it matches '':blank''.
+			(Such as HTML’s <code>&lt;input type=text></code> when its value is empty.)
+		* If it sometimes submits, and is set to not submit, it matches '':blank''.
+			(Such as HTML’s <code>&lt;input type=checkbox></code> when not checked.)
+		* If it's an “action button”
+			(rather than a “toggle button” that represents a state)
+			such as <code>&lt;button></code>, <code>&lt;input type=submit></code>, etc.,
+			it never matches '':blank''.
+
+		Host languages can specify more precise rules
+		for when form controls match '':blank''.
 	</div>
 
 	Note: This selector is at-risk.