diff --git a/core/modules/startup/css-escape-polyfill.js b/core/modules/startup/css-escape-polyfill.js
new file mode 100644
index 000000000..288a7336b
--- /dev/null
+++ b/core/modules/startup/css-escape-polyfill.js
@@ -0,0 +1,114 @@
+/*\
+title: $:/core/modules/startup/CSSescape.js
+type: application/javascript
+module-type: startup
+
+Polyfill for CSS.escape()
+
+\*/
+(function(root,factory){
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+// Export name and synchronous status
+exports.name = "css-escape";
+exports.platforms = ["browser"];
+exports.after = ["startup"];
+exports.synchronous = true;
+
+/*! https://mths.be/cssescape v1.5.1 by @mathias | MIT license */
+// https://github.com/umdjs/umd/blob/master/returnExports.js
+exports.startup = factory(root);
+}(typeof global != 'undefined' ? global : this, function(root) {
+
+	if (root.CSS && root.CSS.escape) {
+		return;
+	}
+
+	// https://drafts.csswg.org/cssom/#serialize-an-identifier
+	var cssEscape = function(value) {
+		if (arguments.length == 0) {
+			throw new TypeError('`CSS.escape` requires an argument.');
+		}
+		var string = String(value);
+		var length = string.length;
+		var index = -1;
+		var codeUnit;
+		var result = '';
+		var firstCodeUnit = string.charCodeAt(0);
+		while (++index < length) {
+			codeUnit = string.charCodeAt(index);
+			// Note: there’s no need to special-case astral symbols, surrogate
+			// pairs, or lone surrogates.
+
+			// If the character is NULL (U+0000), then the REPLACEMENT CHARACTER
+			// (U+FFFD).
+			if (codeUnit == 0x0000) {
+				result += '\uFFFD';
+				continue;
+			}
+
+			if (
+				// If the character is in the range [\1-\1F] (U+0001 to U+001F) or is
+				// U+007F, […]
+				(codeUnit >= 0x0001 && codeUnit <= 0x001F) || codeUnit == 0x007F ||
+				// If the character is the first character and is in the range [0-9]
+				// (U+0030 to U+0039), […]
+				(index == 0 && codeUnit >= 0x0030 && codeUnit <= 0x0039) ||
+				// If the character is the second character and is in the range [0-9]
+				// (U+0030 to U+0039) and the first character is a `-` (U+002D), […]
+				(
+					index == 1 &&
+					codeUnit >= 0x0030 && codeUnit <= 0x0039 &&
+					firstCodeUnit == 0x002D
+				)
+			) {
+				// https://drafts.csswg.org/cssom/#escape-a-character-as-code-point
+				result += '\\' + codeUnit.toString(16) + ' ';
+				continue;
+			}
+
+			if (
+				// If the character is the first character and is a `-` (U+002D), and
+				// there is no second character, […]
+				index == 0 &&
+				length == 1 &&
+				codeUnit == 0x002D
+			) {
+				result += '\\' + string.charAt(index);
+				continue;
+			}
+
+			// If the character is not handled by one of the above rules and is
+			// greater than or equal to U+0080, is `-` (U+002D) or `_` (U+005F), or
+			// is in one of the ranges [0-9] (U+0030 to U+0039), [A-Z] (U+0041 to
+			// U+005A), or [a-z] (U+0061 to U+007A), […]
+			if (
+				codeUnit >= 0x0080 ||
+				codeUnit == 0x002D ||
+				codeUnit == 0x005F ||
+				codeUnit >= 0x0030 && codeUnit <= 0x0039 ||
+				codeUnit >= 0x0041 && codeUnit <= 0x005A ||
+				codeUnit >= 0x0061 && codeUnit <= 0x007A
+			) {
+				// the character itself
+				result += string.charAt(index);
+				continue;
+			}
+
+			// Otherwise, the escaped character.
+			// https://drafts.csswg.org/cssom/#escape-a-character
+			result += '\\' + string.charAt(index);
+
+		}
+		return result;
+	};
+
+	if (!root.CSS) {
+		root.CSS = {};
+	}
+
+	root.CSS.escape = cssEscape;
+
+}));