[css-lists-3] Slightly rephrase the 'implicit list-item counter' section to be clearer, particularly about how UAs can override the increment based on their own semantics.

Author: tabatkins

css-lists-3/Overview.bs

@@ -1063,13 +1063,29 @@ Counters in elements that do not generate boxes</h3>
 <h3 id="list-item-counter">
 The Implicit ''list-item'' Counter</h3>
 
-	<a>List items</a> automatically increment
+	In addition to any explicitly defined [=counters=]
+	that authors write in their styles,
+	[=list items=] automatically increment
 	a special <dfn value for="counter-increment, counter-set, counter-reset, counter(), counters()">list-item</dfn> <a>counter</a>.
-	Unless the 'counter-increment' property explicitly specifies
-	a different increment for the ''list-item'' <a>counter</a>,
-	it must be incremented by 1 on every <a>list item</a>,
-	at the same time that <a>counters</a> are normally incremented.
-	(This has no effect on the computed values of the 'counter-*' properties.)
+
+	By default, the ''list-item'' counter
+	must be incremented by 1 on every [=list item=],
+	exactly as if the [=list item=] had ''list-item 1'' appended to their 'counter-increment' value
+	(including side-effects such as possibly [=instantiating=] a new [=counter=], etc).
+	This happens at the same time as when [=counters=] are normally incremented,
+	and does not affect the [=specified value|specified=] or [=computed value=] of 'counter-increment'.
+
+	This automatic increment <strong>does not</strong> happen
+	if the [=list item’s=] 'counter-increment'
+	explicitly mentions the ''list-item'' counter:
+	''li { counter-increment: list-item 2; }''
+	will increment ''list-item'' by 2,
+	not by 3 as would happen if you additionally appended ''list-item 1'' to that.
+
+	Host languages can additionally define more complex behavior
+	based on their own semantics.
+	For example, HTML lists can have a <{ol/reversed}> attribute,
+	which causes the ''list-item'' counter to count <em>down</em> instead on the list items belonging to that element.
 
 	<div class="example">
 		In the following example,
@@ -1085,28 +1101,29 @@ The Implicit ''list-item'' Counter</h3>
 		</pre>
 	</div>
 
-	Issue: More definition.
+	Note: This means that ''counter-increment: none''
+	still lets the ''list-item'' counter increment on [=list items=].
+	If one wants to explicitly turn off the ''list-item'' counter as well
+	(perhaps because they are doing something more complicated with it themselves),
+	one must write ''counter-increment: list-item 0;''.
+
+	<!--
 	See old <a href="https://github.com/w3c/csswg-drafts/issues/2464#issuecomment-375492907">WG discussions</a>
 	and newer <a href="https://github.com/w3c/csswg-drafts/issues/3686">issue discussion</a>.
-
-	UAs and host languages should ensure
-	that the ''counter-increment/list-item'' counter values
-	by default
-	reflect the underlying numeric value dictated by host language semantics
-	when setting up list item styling
-	in their UA style sheet and presentational hint style mappings.
-	<span class="issue">Not really sure how to word this requirement,
-	but the following example is expected to work.</span>
+	-->
 
 	<div class="example">
 		In the following example,
 		the 'content' property is used to create tiered numbering
 		that hooks into the ''counter-increment/list-item'' counter,
 		and thus respects any numbering changes inflicted through HTML:
+
 		<pre>
 		  ol > li::marker { content: counters(list-item,'.') '.'; }
 		</pre>
+
 		Nested lists using this rule would be rendered like
+
 		<pre>
 		  1. First top-level item
 		  5. Second top-level item, value=5
@@ -1119,23 +1136,26 @@ The Implicit ''list-item'' Counter</h3>
 		     5.5. Third second-level item, list start=3
 		  6. Third top-level item
 		</pre>
+
 		given markup such as
+
 		<xmp>
-		  <ol>
-		    <li>First top-level item
-		    <li value=5>Second top-level item, value=5
-		      <ol start=3>
-		        <li>First second-level item, list start=3
-		        <li>Second second-level item, list start=3
-		           <ol reversed>
-		             <li>First third-level item in reversed list
-		             <li>Second third-level item in reversed list
-		             <li>Third third-level item in reversed list
-		             <li>Fourth third-level item in reversed list
-		           </ol>
-		        <li>Third second-level item, list start=3
-		    <li>Third top-level item
-		  </ol>
+			<ol>
+				<li>First top-level item
+				<li value=5>Second top-level item, value=5
+					<ol start=3>
+						<li>First second-level item, list start=3
+						<li>Second second-level item, list start=3
+							<ol reversed>
+								<li>First third-level item in reversed list
+								<li>Second third-level item in reversed list
+								<li>Third third-level item in reversed list
+								<li>Fourth third-level item in reversed list
+							</ol>
+					</ol>
+				<li>Third second-level item, list start=3
+				<li>Third top-level item
+			</ol>
 		</xmp>
 	</div>