Use numeric font weights instead of keywords.

When Open Sans was in use, the `300`, `400`, and `600` weights were loaded. `400` is the equivalent of `normal`; however, `bold` is equivalent to `700`, not `600`. With the move to system fonts, we need to be specific rather than relying on the lack of a `700` weight. Not all system fonts include a `600` weight; in those instances, they will use the `bold`/`700` weight.

The WordPress CSS Coding Standards have been updated accordingly.

props coderste.
see #36753.

Built from https://develop.svn.wordpress.org/trunk@37740


git-svn-id: http://core.svn.wordpress.org/trunk@37705 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/dashboard-rtl.css

@@ -114,7 +114,7 @@
 .welcome-panel h2 {
 	margin: 0;
 	font-size: 21px;
-	font-weight: normal;
+	font-weight: 400;
 	line-height: 1.2;
 }
 
@@ -342,7 +342,7 @@ body #dashboard-widgets .postbox form .submit {
 }
 
 #dashboard_primary .rss-widget a {
-	font-weight: normal;
+	font-weight: 400;
 }
 
 #dashboard_primary .rss-widget span,
@@ -400,7 +400,7 @@ body #dashboard-widgets .postbox form .submit {
 	margin: 0 0 10px;
 	padding: 0;
 	font-size: 14px;
-	font-weight: normal;
+	font-weight: 400;
 }
 
 /* Dashboard right now - Colors */
@@ -534,7 +534,7 @@ form.initial-form.quickpress-open input#title {
 }
 
 #dashboard_primary a.rsswidget {
-	font-weight: normal;
+	font-weight: 400;
 }
 
 #dashboard_quick_press .drafts ul {
@@ -570,7 +570,7 @@ form.initial-form.quickpress-open input#title {
 	margin: 0 12px 8px;
 	padding: 0;
 	font-size: 14px;
-	font-weight: normal;
+	font-weight: 400;
 	color: #23282d;
 }
 
@@ -829,7 +829,7 @@ form.initial-form.quickpress-open input#title {
 
 #latest-comments #the-comment-list .comment-meta cite {
 	font-style: normal;
-	font-weight: normal;
+	font-weight: 400;
 }
 
 #latest-comments #the-comment-list .comment-item blockquote,