Merge pull request #17522 from twbs/v4-dev-xmr-ie9-gradient

docs: IE9 gradient.

docs/assets/scss/_ads.scss

@@ -13,7 +13,7 @@
   line-height: 1rem !important;
   color: $bd-purple-light !important;
   text-align: left;
-  background: darken($bd-purple, 10%) !important;
+  background-color: darken($bd-purple, 10%) !important;
   border: 0 !important;
 }
 

docs/assets/scss/_masthead.scss

@@ -3,8 +3,9 @@
   padding: 3rem ($grid-gutter-width / 2) 2rem;
   color: $bd-purple-light;
   text-align: center;
-  background: -webkit-linear-gradient(135deg, darken($bd-purple, 20%), $bd-purple, lighten(saturate($bd-purple, 5%), 15%));
-  background:         linear-gradient(135deg, darken($bd-purple, 20%), $bd-purple, lighten(saturate($bd-purple, 5%), 15%));
+  background-image: -webkit-linear-gradient(135deg, darken($bd-purple, 20%), $bd-purple, lighten(saturate($bd-purple, 5%), 15%));
+  background-image:         linear-gradient(135deg, darken($bd-purple, 20%), $bd-purple, lighten(saturate($bd-purple, 5%), 15%));
+  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#{ie-hex-str(darken($bd-purple, 20%))}', endColorstr='#{ie-hex-str(lighten(saturate($bd-purple, 5%), 15%))}', gradientType='1');   /* For IE9 */
 
   .bd-booticon {
     margin: 0 auto 2rem;