Include skip link JS for IE. (#937)

* Include skip link JS for IE.

* Output minified script without enqueuing it

Fixes #935

assets/js/skip-link-focus-fix.js

@@ -0,0 +1,33 @@
+/**
+ * File skip-link-focus-fix.js.
+ *
+ * Helps with accessibility for keyboard only users.
+ *
+ * This is the source file for what is minified in the twentytwenty_skip_link_focus_fix() PHP function.
+ *
+ * Learn more: https://git.io/vWdr2
+ */
+( function() {
+	var isIe = /(trident|msie)/i.test( navigator.userAgent );
+
+	if ( isIe && document.getElementById && window.addEventListener ) {
+		window.addEventListener( 'hashchange', function() {
+			var id = location.hash.substring( 1 ),
+				element;
+
+			if ( ! ( /^[A-z0-9_-]+$/.test( id ) ) ) {
+				return;
+			}
+
+			element = document.getElementById( id );
+
+			if ( element ) {
+				if ( ! ( /^(?:a|select|input|button|textarea)$/i.test( element.tagName ) ) ) {
+					element.tabIndex = -1;
+				}
+
+				element.focus();
+			}
+		}, false );
+	}
+}() );

functions.php

@@ -209,15 +209,32 @@ function twentytwenty_register_scripts() {
 add_action( 'wp_enqueue_scripts', 'twentytwenty_register_scripts' );
 
 /**
- * Enqueue non-latin language styles
+ * Fix skip link focus in IE11.
+ *
+ * This does not enqueue the script because it is tiny and because it is only for IE11,
+ * thus it does not warrant having an entire dedicated blocking script being loaded.
+ *
+ * @link https://git.io/vWdr2
+ */
+function twentytwenty_skip_link_focus_fix() {
+	// The following is minified via `terser --compress --mangle -- assets/js/skip-link-focus-fix.js`.
+	?>
+	<script>
+	/(trident|msie)/i.test(navigator.userAgent)&&document.getElementById&&window.addEventListener&&window.addEventListener("hashchange",function(){var t,e=location.hash.substring(1);/^[A-z0-9_-]+$/.test(e)&&(t=document.getElementById(e))&&(/^(?:a|select|input|button|textarea)$/i.test(t.tagName)||(t.tabIndex=-1),t.focus())},!1);
+	</script>
+	<?php
+}
+add_action( 'wp_print_footer_scripts', 'twentytwenty_skip_link_focus_fix' );
+
+/** Enqueue non-latin language styles
  *
  * @since 1.0.0
  *
  * @return void
  */
 function twentytwenty_non_latin_languages() {
 	$custom_css = TwentyTwenty_Non_Latin_Languages::get_non_latin_css( 'front-end' );
-
+	
 	if ( $custom_css ) {
 		wp_add_inline_style( 'twentytwenty-style', $custom_css );
 	}