diff --git a/src/components/content.js b/src/components/content.js
index f3c8b7540..091580be6 100644
--- a/src/components/content.js
+++ b/src/components/content.js
@@ -3,6 +3,7 @@ import Base64 from '../serializers/base-64'
 import Debug from 'debug'
 import Node from './node'
 import getPoint from '../utils/get-point'
+import findClosestNode from '../utils/find-closest-node'
 import React from 'react'
 import Selection from '../models/selection'
 import getTransferData from '../utils/get-transfer-data'
@@ -243,7 +244,7 @@ class Content extends React.Component {
     const { target } = event
     return (
       (target.isContentEditable) &&
-      (target == element || target.closest('[data-slate-editor]') == element)
+      (target == element || findClosestNode(target, '[data-slate-editor]') == element)
     )
   }
 
diff --git a/src/utils/find-closest-node.js b/src/utils/find-closest-node.js
new file mode 100644
index 000000000..f55177acf
--- /dev/null
+++ b/src/utils/find-closest-node.js
@@ -0,0 +1,32 @@
+
+/**
+ * Find the closest ancestor of a DOM `element` that matches a given selector.
+ *
+ * @param {Element} node
+ * @param {String} selector
+ * @return {Element}
+ */
+
+function findClosestNode(node, selector) {
+  if (typeof node.closest === 'function') return node.closest(selector)
+
+  // See https://developer.mozilla.org/en-US/docs/Web/API/Element/closest#Polyfill
+  const matches = (node.document || node.ownerDocument).querySelectorAll(selector)
+  let i
+  let parentNode = node
+  do {
+    i = matches.length
+    while (--i >= 0 && matches.item(i) !== parentNode);
+  }
+  while ((i < 0) && (parentNode = parentNode.parentElement))
+
+  return parentNode
+}
+
+/**
+ * Export.
+ *
+ * @type {Function}
+ */
+
+export default findClosestNode
diff --git a/src/utils/offset-key.js b/src/utils/offset-key.js
index c5ca6e34e..4fbc5ec0d 100644
--- a/src/utils/offset-key.js
+++ b/src/utils/offset-key.js
@@ -1,5 +1,6 @@
 
 import normalizeNodeAndOffset from './normalize-node-and-offset'
+import findClosestNode from './find-closest-node'
 
 /**
  * Offset key parser regex.
@@ -68,12 +69,12 @@ function findKey(rawNode, rawOffset) {
   const { parentNode } = node
 
   // Find the closest parent with an offset key attribute.
-  let closest = parentNode.closest(SELECTOR)
+  let closest = findClosestNode(parentNode, SELECTOR)
 
   // For void nodes, the element with the offset key will be a cousin, not an
   // ancestor, so find it by going down from the nearest void parent.
   if (!closest) {
-    const closestVoid = parentNode.closest(VOID_SELECTOR)
+    const closestVoid = findClosestNode(parentNode, VOID_SELECTOR)
     if (!closestVoid) return null
     closest = closestVoid.querySelector(SELECTOR)
     offset = closest.textContent.length