avoid unnecessary HTML string building+parsing

js/modal.js

@@ -190,7 +190,8 @@
     if (this.isShown && this.options.backdrop) {
       var doAnimate = $.support.transition && animate
 
-      this.$backdrop = $('<div class="modal-backdrop ' + animate + '" />')
+      this.$backdrop = $(document.createElement('div'))
+        .addClass('modal-backdrop ' + animate)
         .appendTo(this.$body)
 
       this.$element.on('click.dismiss.bs.modal', $.proxy(function (e) {