Merge pull request #18081 from twbs/ie9-click-comment

Give better description of IE9 transparent click bug & workaround

scss/_carousel.scss

@@ -178,9 +178,9 @@
     cursor: pointer;
     // IE9 hack for event handling
     //
-    // Internet Explorer 9 does not support clicks on elements without a set
-    // `background-color`. We cannot use `filter` since that's not viewed as a
-    // background color by the browser. Thus, a hack is needed.
+    // Internet Explorer 9 does not properly handle clicks on elements with a `background-color` of `transparent`,
+    // so we use rgba(0,0,0,0) instead since it's a non-buggy equivalent.
+    // See https://developer.mozilla.org/en-US/docs/Web/Events/click#Internet_Explorer
     background-color: rgba(0,0,0,0); // IE9
     border: 1px solid $carousel-indicator-border-color;
     border-radius: 10px;