[css-grid-2] Fix errors in subgrid gap examples, add image for normal. p=Dan503 #2285 (comment)

Author: fantasai

css-grid-2/Overview.bs

@@ -265,7 +265,7 @@ Characteristics of a Subgrid Item</h3>
 				A subgrid spanning both tracks would have&hellip;
 
 				<dl>
-					<dt>&hellip; if its 'column-gap' were ''column-gap/normal'':
+					<dt>&hellip; if its 'column-gap' were ''column-gap/normal'' (or ''50px''):
 					<dd>
 						<ul>
 							<li>A grid item in its left column
@@ -279,13 +279,15 @@ Characteristics of a Subgrid Item</h3>
 								sized and laid out
 								(and contributing its size to the parent grid’s sizing calculations)
 								without any special adjustment,
-								thus stretching to 175px wide,
+								thus stretching to 150px wide,
 								while remaining aligned
 								to the subgrid’s right edge.
 							<li>An effective visual <a>gutter</a> between the items
 								of 50px, exactly matching its parent grid.
 						</ul>
 
+						<img src="images/subgrid-gap-normal.png" alt="">
+
 					<dt>&hellip; if its 'column-gap' were ''0'':
 					<dd>
 						<ul>
@@ -338,14 +340,14 @@ Characteristics of a Subgrid Item</h3>
 							<li>A grid item in its left column
 								sized and laid out
 								(and contributing its size to the parent grid’s sizing calculations)
-								as if it had a -37.5px right margin,
+								as if it had a 12.5px right margin,
 								thus stretching to 87.5px wide
 								while remaining aligned
 								to the subgrid’s left edge.
 							<li>A grid item in its right column
 								sized and laid out
 								(and contributing its size to the parent grid’s sizing calculations)
-								as if it had a -37.5px left margin,
+								as if it had a 12.5px left margin,
 								thus stretching to 137.5px wide,
 								while remaining aligned
 								to the subgrid’s right edge.