[css-backgrounds-3][editorial] Fix bikeshed errors and warnings (#11912)

Author: kbabbitt

css-backgrounds-3/Overview.bs

@@ -28,7 +28,7 @@ spec:css2; type:value; text:table
 spec:css2; type:value; text:inline-table
 spec:css2; type:value; text:table-cell
 spec:css2; type:property; text:overflow
-spec:css2; type:value; text:visible
+spec:css2; type:value; for:overflow; text:visible
 spec:css2; type:dfn; text:viewport
 spec:css-break-4; type:dfn; text:fragment
 spec:css-color-4; type:property; text:color
@@ -832,7 +832,7 @@ Affixing Images: the 'background-attachment' property</h3>
 					<figcaption>
 						Diagram of the meaning of ''background-position: 75% 50%''.
 					</figcaption>
-				</div>
+				</figure>
 			</div>
 
 		<dt><dfn><<length>></dfn>
@@ -2602,7 +2602,7 @@ Overlapping Curves</h3>
 
 			<figcaption>
 				These rounded corner might be the result of
-				'width: 6em; height: 2.5em; border-radius: 0.5em 2em 0.5em 2em''
+				''width: 6em; height: 2.5em; border-radius: 0.5em 2em 0.5em 2em''
 				for A; and ditto but with ''height: 2em'' for B.
 			</figcaption>
 		</figure>