Add compatible css centered demo code, close #3849

components/modal/demo/manual.md

@@ -1,6 +1,7 @@
 ---
 order: 7
-title: 
+debug: true
+title:
   zh-CN: 手动移除
   en-US: Manual to destroy
 ---

components/modal/demo/position.md

@@ -1,6 +1,6 @@
 ---
 order: 7
-title: 
+title:
   zh-CN: 自定义位置
   en-US: To customize the position of modal
 ---
@@ -68,6 +68,28 @@ ReactDOM.render(<App />, mountNode);
 
 ````css
 /* use css to set position of modal */
+.vertical-center-modal {
+  text-align: center;
+  white-space: nowrap;
+}
+
+.vertical-center-modal:before {
+  content: '';
+  display: inline-block;
+  height: 100%;
+  vertical-align: middle;
+  width: 0;
+}
+
+.vertical-center-modal .ant-modal {
+  display: inline-block;
+  vertical-align: middle;
+  top: 0;
+  text-align: left;
+}
+
+/*
+// Use flex which not working in IE
 .vertical-center-modal {
   display: flex;
   align-items: center;
@@ -77,4 +99,5 @@ ReactDOM.render(<App />, mountNode);
 .vertical-center-modal .ant-modal {
   top: 0;
 }
+*/
 ````