Fix examples' issues #25664 and #25388 (#25665)

* Prevent sign-in form from going outside viewport (#25388)

* Workaround IE centering bug

docs/4.0/examples/floating-labels/floating-labels.css

@@ -13,11 +13,8 @@ body {
   display: -webkit-box;
   display: flex;
   -ms-flex-align: center;
-  -ms-flex-pack: center;
   -webkit-box-align: center;
   align-items: center;
-  -webkit-box-pack: center;
-  justify-content: center;
   padding-top: 40px;
   padding-bottom: 40px;
   background-color: #f5f5f5;
@@ -27,7 +24,7 @@ body {
   width: 100%;
   max-width: 420px;
   padding: 15px;
-  margin: 0 auto;
+  margin: auto;
 }
 
 .form-label-group {