diff --git a/core/themes/stable/css/core/splitbutton/splitbutton.css b/core/themes/stable/css/core/splitbutton/splitbutton.css
new file mode 100644
index 0000000000..411301b33b
--- /dev/null
+++ b/core/themes/stable/css/core/splitbutton/splitbutton.css
@@ -0,0 +1,124 @@
+
+/**
+ * @file
+ * Base styles for splitbuttons.
+ */
+
+/**
+ * When a splitbutton has only one item, it is simply a button.
+ */
+.splitbutton {
+  display: inline-block;
+  box-sizing: border-box;
+  max-width: 100%;
+}
+
+.splitbutton--multiple {
+  padding-right: 2em; /* LTR */
+}
+
+[dir="rtl"] .splitbutton--multiple {
+  padding-right: 0;
+  padding-left: 2em;
+}
+
+.js .splitbutton--multiple {
+  position: relative;
+}
+
+.splitbutton__action.splitbutton__action {
+  width: 100%;
+  margin: 0;
+  text-align: left; /* LTR */
+}
+[dir="rtl"] .splitbutton__action.splitbutton__action {
+  text-align: right;
+}
+
+@media screen and (max-width: 600px) {
+  .js .splitbutton {
+    width: 100%;
+  }
+}
+
+.js td .splitbutton-multiple .splitbutton-action a,
+.js td .splitbutton-multiple .splitbutton-action input,
+.js td .splitbutton-multiple .splitbutton-action button {
+  width: auto;
+}
+
+/* UL styles are over-scoped in core, so this selector needs weight parity. */
+.splitbutton__list.splitbutton__list {
+  margin: 0;
+  padding: 0;
+  list-style: none;
+}
+
+.js .splitbutton__list .splitbutton__action {
+  display: none;
+}
+
+.splitbutton__list.open .splitbutton__action {
+  display: block;
+}
+
+/**
+ * The splitbutton styling.
+ *
+ * A splitbutton is a widget that displays a list of action links as a button
+ * with a primary action. Secondary actions are hidden behind a click on a
+ * twisty arrow.
+ *
+ * The arrow is created using border on a zero-width, zero-height span.
+ * The arrow inherits the link color, but can be overridden with border colors.
+ */
+.splitbutton.open {
+  z-index: 100;
+  max-width: none;
+}
+
+.splitbutton__toggle.splitbutton__toggle {
+  position: absolute;
+  top: 0;
+  right: 0; /* LTR */
+  bottom: 0;
+  width: 2em;
+  margin: 0;
+  padding: 0;
+}
+[dir="rtl"] .splitbutton__toggle.splitbutton__toggle {
+  right: auto;
+  left: 0;
+}
+
+.splitbutton__toggle-arrow {
+  position: absolute;
+  top: 50%;
+  left: 50%;
+  width: 0;
+  height: 0;
+  transform: translate(-50%, -50%);
+  border-width: 0.3333em 0.3333em 0;
+  border-style: solid;
+  border-right-color: transparent;
+  border-bottom-color: transparent;
+  border-left-color: transparent;
+  line-height: 0;
+}
+
+.splitbutton.open .splitbutton__toggle-arrow {
+  border-top-color: transparent;
+  border-bottom: 0.3333em solid;
+}
+
+.splitbutton .ajax-progress-throbber {
+  position: absolute;
+  top: 100%;
+  left: 0;
+  display: flex;
+  width: auto;
+}
+
+.splitbutton .ajax-progress-throbber .message {
+  flex: 1 0 auto;
+}
diff --git a/core/themes/stable/stable.info.yml b/core/themes/stable/stable.info.yml
index 0f20f7c604..608db4af6e 100644
--- a/core/themes/stable/stable.info.yml
+++ b/core/themes/stable/stable.info.yml
@@ -85,6 +85,13 @@ libraries-override:
     css:
       component:
         misc/dropbutton/dropbutton.css: css/core/dropbutton/dropbutton.css
+
+  core/drupal.splitbutton:
+    css:
+      component:
+        misc/splitbutton/splitbutton.css: css/core/splitbutton/splitbutton.css
+
+
   core/drupal.vertical-tabs:
     css:
       component: