[css-shapes] editorial clarifications

Author: astearns

css-shapes/Overview.html

@@ -490,6 +490,10 @@ <h3 class="heading settled heading" data-level=3.1 id=supported-basic-shapes><sp
 					of the ellipse, 
 					in that order. 
 					Negative values for either radius are invalid. 
+					Percentage values here are resolved 
+					against the used width (for the rx value) 
+					and the used height (for the ry value) 
+					of the reference box.
 				</li>
 				<li>
 					The position argument defines 
@@ -629,6 +633,9 @@ <h2 class="heading settled heading" data-level=4 id=shapes-from-image><span clas
 	that enclose the area(s) 
 	where the opacity of the specified image 
 	is greater than the <a class=property data-link-type=propdesc href=#propdef-shape-image-threshold title=shape-image-threshold>shape-image-threshold</a> value. 
+	The absence of any pixels with an alpha value 
+	larger than the specified threshold 
+	results in an <a data-link-type=dfn href=#empty-float-area title="empty float area">empty float area</a>.
 	If the <a class=property data-link-type=propdesc href=#propdef-shape-image-threshold title=shape-image-threshold>shape-image-threshold</a> is not specified, 
 	the initial value to be considered is 0.0.
 

css-shapes/Overview.src.html

@@ -371,6 +371,10 @@ <h3 id="supported-basic-shapes">
 					of the ellipse, 
 					in that order. 
 					Negative values for either radius are invalid. 
+					Percentage values here are resolved 
+					against the used width (for the rx value) 
+					and the used height (for the ry value) 
+					of the reference box.
 				</li>
 				<li>
 					The position argument defines 
@@ -510,6 +514,9 @@ <h2 id="shapes-from-image">
 	that enclose the area(s) 
 	where the opacity of the specified image 
 	is greater than the 'shape-image-threshold' value. 
+	The absence of any pixels with an alpha value 
+	larger than the specified threshold 
+	results in an <a>empty float area</a>.
 	If the 'shape-image-threshold' is not specified, 
 	the initial value to be considered is 0.0.