[css-writing-modes] Specify MS Gothic for example, since it shows off the codepoint differences better than the default font.

Author: fantasai

css-writing-modes/Overview.bs

@@ -2661,7 +2661,7 @@ Full-width Characters</h5>
     it will be transformed to a fullwidth "２".
     Since the "2010" was not combined, its digits, too, will be transformed to fullwidth "２０１０";
     and being fullwidth, they will be typeset upright, giving the following result:
-    <pre style="text-align: center">
+    <pre style="text-align: center; font-family: MS Gothic, sans-serif">
 <!-- -->２
 <!-- -->０
 <!-- -->１