Update Overview.bs

Author: xiaochengh

css-fonts-4/Overview.bs

@@ -2933,26 +2933,29 @@ For: @font-face
 Initial: normal
 </pre>
 
-These 'ascent-override', 'descent-override' and 'line-gap-override' descriptors define the values
-of the ascent, descent and line gap metrics of the font.
+The 'ascent-override', 'descent-override' and 'line-gap-override' descriptors define the
+<a spec="CSS-INLINE-3">ascent metric</a>, <a spec="CSS-INLINE-3">descent metric</a> and
+<a spec="CSS-INLINE-3">line gap metric</a> of the font.
 
-When the descriptor value is 'normal', the metric value is obtained from the font file directly.
+When the descriptor value is 'normal', the corresponding metric value is obtained from the
+font file directly.
 
 Note: User agents may choose different pieces of data from the font file as the metric value,
 leading to different layouts. For example, Chrome on Windows uses sTypoAscender, sTypoDescender
-and sTypoLineGap from the font's OS/2 table, while on Mac, it uses those in the HHEA table if available.
+and sTypoLineGap from the font's OS/2 table, while on Mac, it uses those in the HHEA table if
+available.
 
-When the descriptor value is a percentage, the metric value is set to the given percentage of the
-em size of the font.
+When the descriptor value is a percentage, the corresponding metric value is resolved as the
+given percentage multiplied by the used font size. Negative values are illegal.
 
 <div class="example">
 	We may override the metrics of Arial as follows:
 
 <pre>@font-face {
   font-family: arial;
   src: local(Arial), url("http://example.com/arial.woff");
-  ascent-override: 60%;
-  descent-override: 40%;
+  ascent-override: 80%;
+  descent-override: 20%;
   line-gap-override: 0%;
 }
 </pre>
@@ -2965,7 +2968,7 @@ font-size: 20px;
 line-height: normal;
 </pre>
 
-	Each line box will be exactly 20px high. Text baseline will be positioned at 12px
+	Each line box will be exactly 20px high. Text baseline will be positioned at 16px
 	below line box top.
 
 </div>