Add flexbox support for webkit and IE.

src/grids/css/grids-r.css

@@ -22,6 +22,18 @@
     * Helvetica, Arial, sans-serif: Common font stack on OS X and Windows.
     */
     font-family: FreeSans, Arimo, "Droid Sans", Helvetica, Arial, sans-serif;
+
+    /* Use flexbox when possible to avoid letter-spacing side-effects.
+       NOTE: Firefox (as of 25) does not currently support flex-wrap,
+       so the -moz- version is omitted.
+    */
+
+    display: -webkit-flex;
+    -webkit-flex-flow: row wrap;
+
+    /* IE10 uses display: flexbox */
+    display: -ms-flexbox;
+    -ms-flex-flow: row wrap;
 }
 
 /* Opera as of 12 on Windows needs word-spacing.