[css-background-3][css-easing][css-shapes-1] Rewrite syntax with implicitly optional comma (w3c#9650)

Author: cdoublev

css-backgrounds-3/Overview.bs

@@ -970,7 +970,7 @@ layer.
 
 <pre class="propdef">
 Name: background
-Value: [<<bg-layer>># ,]? <<final-bg-layer>>
+Value: <<bg-layer>>#? , <<final-bg-layer>>
 Initial: see individual properties
 Applies to: all elements
 Inherited: no

css-easing-1/Overview.bs

@@ -249,8 +249,8 @@ It has following syntax:
 <div class="prod">
   <dfn type>&lt;step-easing-function&gt;</dfn> =
   ''step-start'' | ''step-end'' |
-  <span class="atom"><a lt="steps()" function>steps</a>(<<integer>>[,
-    <<step-position>>]?)</span>
+  <span class="atom"><a lt="steps()" function>steps</a>(<<integer>> ,
+    <<step-position>>?)</span>
 
   <dfn type>&lt;step-position&gt;</dfn> =
     ''jump-start'' | ''jump-end'' |

css-easing-2/Overview.bs

@@ -563,8 +563,8 @@ It has following syntax:
 <div class="prod">
   <dfn type>&lt;step-easing-function&gt;</dfn> =
   ''step-start'' | ''step-end'' |
-  <span class="atom"><a lt="steps()" function>steps</a>(<<integer>>[,
-    <<step-position>>]?)</span>
+  <span class="atom"><a lt="steps()" function>steps</a>(<<integer>> ,
+    <<step-position>>?)</span>
 
   <dfn type>&lt;step-position&gt;</dfn> =
     ''jump-start'' | ''jump-end'' |

css-shapes-1/Overview.bs

@@ -497,7 +497,7 @@ Supported Shapes</h3>
 
 		</dd>
 		<dt><dfn>path()</dfn> = 
-			path( [<<'fill-rule'>>,]? <<string>> )
+			path( <<'fill-rule'>>? , <<string>> )
 		</dt>
 		<dd dfn-type=value dfn-for="path()">
 			<ul>