doc: fix highlighting control elements for Webkit touch (#14703)

examples/assets/styles/common.css

@@ -5,6 +5,7 @@ html, body {
   font-family: 'Helvetica Neue',Helvetica,'PingFang SC','Hiragino Sans GB','Microsoft YaHei',SimSun,sans-serif;
   font-weight: 400;
   -webkit-font-smoothing: antialiased;
+  -webkit-tap-highlight-color: transparent;
 
   &.is-component {
     overflow: hidden;