diff --git a/mu-plugins/helpers/helpers.php b/mu-plugins/helpers/helpers.php
index 78e8d4fc..91c06dbf 100644
--- a/mu-plugins/helpers/helpers.php
+++ b/mu-plugins/helpers/helpers.php
@@ -1,6 +1,7 @@
 <?php
 
 namespace WordPressdotorg\MU_Plugins\Helpers;
+
 defined( 'WPINC' ) || die();
 
 /**
@@ -22,3 +23,30 @@ function natural_language_join( array $list, $conjunction = 'and' ) : string {
 
 	return $last;
 }
+
+/**
+ * Check if a plugin is active on the current site.
+ *
+ * Clone of core's `is_plugin_active` and `is_plugin_active_for_network`, so
+ * they can be used on the frontend.
+ *
+ * @param string $plugin Path to the plugin file relative to the plugins directory.
+ * @return bool True, if in the active plugins list. False, not in the list.
+ */
+function is_plugin_active( $plugin ) {
+	// Single-site active.
+	if ( in_array( $plugin, (array) get_option( 'active_plugins', array() ), true ) ) {
+		return true;
+	}
+
+	if ( ! is_multisite() ) {
+		return false;
+	}
+
+	$plugins = get_site_option( 'active_sitewide_plugins' );
+	if ( isset( $plugins[ $plugin ] ) ) {
+		return true;
+	}
+
+	return false;
+}
diff --git a/mu-plugins/plugin-tweaks/code-syntax-block/index.php b/mu-plugins/plugin-tweaks/code-syntax-block/index.php
new file mode 100644
index 00000000..14e3dbe5
--- /dev/null
+++ b/mu-plugins/plugin-tweaks/code-syntax-block/index.php
@@ -0,0 +1,148 @@
+<?php
+/**
+ * Code Syntax Block Tweaks
+ *
+ * Adds support for the shortcodes provided by SyntaxHighlighter Evolved.
+ * This allows the current content (with shortcodes) to use the new design's
+ * syntax highlighting style & behavior.
+ *
+ * Also injects the stylesheet with our custom styles.
+ */
+
+namespace WordPressdotorg\MU_Plugins\Plugin_Tweaks\CodeSyntaxBlock;
+
+use function WordPressdotorg\MU_Plugins\Helpers\is_plugin_active;
+
+add_action( 'init', __NAMESPACE__ . '\maybe_init' );
+add_filter( 'mkaz_code_syntax_force_loading', '__return_true' );
+add_filter( 'mkaz_prism_css_url', __NAMESPACE__ . '\update_prism_css_url' );
+
+/**
+ * Initialize the shortcode replacements only if the Code Syntax Block is active.
+ */
+function maybe_init() {
+	if ( ! is_plugin_active( 'code-syntax-block/index.php' ) ) {
+		return;
+	}
+
+	add_shortcode( 'php', __NAMESPACE__ . '\do_shortcode_php' );
+	add_shortcode( 'js', __NAMESPACE__ . '\do_shortcode_js' );
+	add_shortcode( 'css', __NAMESPACE__ . '\do_shortcode_css' );
+	add_shortcode( 'code', __NAMESPACE__ . '\do_shortcode_code' );
+
+	add_filter(
+		'no_texturize_shortcodes',
+		function ( $shortcodes ) {
+			$shortcodes[] = 'php';
+			$shortcodes[] = 'js';
+			$shortcodes[] = 'css';
+			$shortcodes[] = 'code';
+			return $shortcodes;
+		}
+	);
+}
+
+/**
+ * Customize the syntax highlighter style.
+ * See https://github.com/PrismJS/prism-themes.
+ *
+ * @param string $url Absolute URL of the default CSS file you want to enqueue.
+ * @return string
+ */
+function update_prism_css_url( $url ) {
+	return plugins_url( 'prism.css', __FILE__ );
+}
+
+/**
+ * Render the php shortcode using the Code Syntax Block syntax.
+ *
+ * @param array|string $attr    Shortcode attributes array or empty string.
+ * @param string       $content Shortcode content.
+ * @param string       $tag     Shortcode name.
+ * @return string
+ */
+function do_shortcode_php( $attr, $content, $tag ) {
+	$attr = is_array( $attr ) ? $attr : array();
+	$attr['lang'] = 'php';
+
+	return do_shortcode_code( $attr, $content, $tag );
+}
+
+/**
+ * Render the js shortcode using the Code Syntax Block syntax.
+ *
+ * @param array|string $attr    Shortcode attributes array or empty string.
+ * @param string       $content Shortcode content.
+ * @param string       $tag     Shortcode name.
+ * @return string
+ */
+function do_shortcode_js( $attr, $content, $tag ) {
+	$attr = is_array( $attr ) ? $attr : array();
+	$attr['lang'] = 'js';
+
+	return do_shortcode_code( $attr, $content, $tag );
+}
+
+/**
+ * Render the css shortcode using the Code Syntax Block syntax.
+ *
+ * @param array|string $attr    Shortcode attributes array or empty string.
+ * @param string       $content Shortcode content.
+ * @param string       $tag     Shortcode name.
+ * @return string
+ */
+function do_shortcode_css( $attr, $content, $tag ) {
+	$attr = is_array( $attr ) ? $attr : array();
+	$attr['lang'] = 'css';
+
+	return do_shortcode_code( $attr, $content, $tag );
+}
+
+/**
+ * Render the code shortcode using the Code Syntax Block syntax.
+ *
+ * @param array|string $attr    Shortcode attributes array or empty string.
+ * @param string       $content Shortcode content.
+ * @param string       $tag     Shortcode name.
+ * @return string
+ */
+function do_shortcode_code( $attr, $content, $tag ) {
+	// Use an allowedlist of languages, falling back to PHP.
+	// This should account for all languages used in the handbooks.
+	$lang_list = [ 'js', 'json', 'sh', 'bash', 'html', 'css', 'scss', 'php', 'markdown', 'yaml' ];
+	$lang = in_array( $attr['lang'] ?? '', $lang_list ) ? $attr['lang'] ?? '' : 'php';
+
+	$content = _trim_code( $content );
+	// Hides numbers if <= 4 lines of code (last line has no linebreak).
+	$show_line_numbers = substr_count( $content, "\n" ) > 3;
+
+	// Shell is flagged with `sh` or `bash` in the handbooks, but Prism uses `shell`.
+	if ( 'sh' === $lang || 'bash' === $lang ) {
+		$lang = 'shell';
+	}
+
+	return do_blocks(
+		sprintf(
+			'<!-- wp:code {"lineNumbers":%3$s} --><pre class="wp-block-code"><code lang="%1$s" class="language-%1$s %4$s">%2$s</code></pre><!-- /wp:code -->',
+			$lang,
+			$content,
+			$show_line_numbers ? 'true' : 'false',
+			$show_line_numbers ? 'line-numbers' : ''
+		)
+	);
+}
+
+/**
+ * Trim off any extra space, including initial new lines.
+ * Strip out <br /> and <p> added by WordPress.
+ *
+ * @param string $content Shortcode content.
+ * @return string
+ */
+function _trim_code( $content ) {
+	$content = preg_replace( '/<br \/>/', '', $content );
+	$content = preg_replace( '/<\/p>\s*<p>/', "\n\n", $content );
+	// Trim everything except leading spaces.
+	$content = trim( $content, "\n\r\t\v\x00" );
+	return $content;
+}
diff --git a/mu-plugins/plugin-tweaks/code-syntax-block/prism.css b/mu-plugins/plugin-tweaks/code-syntax-block/prism.css
new file mode 100644
index 00000000..96445c8c
--- /dev/null
+++ b/mu-plugins/plugin-tweaks/code-syntax-block/prism.css
@@ -0,0 +1,246 @@
+/**
+ * Custom theme for developer.wordpress.org.
+ * Forked from a11y-dark theme by ericwbailey, which was based on the okaidia theme.
+ *
+ * https://github.com/PrismJS/prism-themes/blob/master/themes/prism-a11y-dark.css
+ * https://github.com/PrismJS/prism/blob/gh-pages/themes/prism-okaidia.css
+ */
+
+code[class*="language-"],
+pre[class*="language-"] {
+	color: #101517;
+	background: none;
+	font-family: Hack, "Fira Code", Consolas, Menlo, Monaco, "Andale Mono", "Lucida Console", "Lucida Sans Typewriter", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Liberation Mono", "Nimbus Mono L", "Courier New", Courier, monospace;
+	text-align: left;
+	white-space: pre;
+	word-spacing: normal;
+	word-break: normal;
+	word-wrap: normal;
+	line-height: 1.5;
+	-moz-tab-size: 4;
+	-o-tab-size: 4;
+	tab-size: 4;
+	-webkit-hyphens: none;
+	-moz-hyphens: none;
+	-ms-hyphens: none;
+	hyphens: none;
+	transition: height 500ms;
+}
+
+/* Code blocks */
+pre[class*="language-"] {
+	padding: 1em;
+	overflow: auto;
+	border-radius: 0.3em;
+}
+
+:not(pre) > code[class*="language-"],
+pre[class*="language-"] {
+	background: #f6f7f7;
+}
+
+/* Inline code */
+:not(pre) > code[class*="language-"] {
+	padding: 0.1em;
+	border-radius: 0.3em;
+	white-space: normal;
+}
+
+.token.comment,
+.token.prolog,
+.token.doctype,
+.token.cdata {
+	color: #007017;
+}
+
+.token.punctuation {
+	color: #50575e;
+}
+
+.token.property,
+.token.tag,
+.token.constant,
+.token.symbol,
+.token.deleted {
+	color: #043959;
+}
+
+.token.boolean,
+.token.number {
+	color: #101517;
+}
+
+.token.selector,
+.token.attr-name,
+.token.string,
+.token.char,
+.token.builtin,
+.token.inserted {
+	color: #2271b1;
+}
+
+.token.operator,
+.token.entity,
+.token.url,
+.language-css .token.string,
+.style .token.string,
+.token.variable {
+	color: #a85f00;
+}
+
+.token.function {
+	color: #b8236d;
+}
+
+.token.keyword {
+	color: #135e96;
+	font-weight: 600;
+}
+
+.token.atrule,
+.token.attr-value,
+.token.function-definition {
+	color: #101517;
+}
+
+.token.important,
+.token.bold {
+	font-weight: 700;
+}
+
+.token.italic {
+	font-style: italic;
+}
+
+.token.entity {
+	cursor: help;
+}
+
+@media screen and (-ms-high-contrast: active) {
+	code[class*="language-"],
+	pre[class*="language-"] {
+		color: windowText;
+		background: window;
+	}
+
+	:not(pre) > code[class*="language-"],
+	pre[class*="language-"] {
+		background: window;
+	}
+
+	.token.important {
+		background: highlight;
+		color: window;
+		font-weight: 700;
+	}
+
+	.token.atrule,
+	.token.attr-value,
+	.token.function,
+	.token.keyword,
+	.token.operator,
+	.token.selector {
+		font-weight: 700;
+	}
+
+	.token.attr-value,
+	.token.comment,
+	.token.doctype,
+	.token.function,
+	.token.keyword,
+	.token.operator,
+	.token.property,
+	.token.string {
+		color: highlight;
+	}
+
+	.token.attr-value,
+	.token.url {
+		font-weight: 700;
+	}
+}
+
+/* Line Numbers */
+pre.line-numbers {
+	position: relative;
+	padding-left: 3.8em;
+	counter-reset: linenumber;
+}
+
+pre.line-numbers > code {
+	position: relative;
+}
+
+.line-numbers .line-numbers-rows {
+	position: absolute;
+	pointer-events: none;
+	top: 0;
+	font-size: 100%;
+	left: -3.8em;
+	width: 4em; /* works for line-numbers below 10000 lines */
+	letter-spacing: -1px;
+	border-right: 0;
+	-webkit-user-select: none;
+	-moz-user-select: none;
+	-ms-user-select: none;
+	user-select: none;
+}
+
+.line-numbers-rows > span {
+	pointer-events: none;
+	display: block;
+	counter-increment: linenumber;
+}
+
+.line-numbers-rows > span::before {
+	content: counter(linenumber);
+	color: #5c6370;
+	display: block;
+	padding-right: 0.8em;
+	text-align: right;
+}
+
+/* Copy button */
+.wp-code-block-button-container {
+	position: sticky;
+	top: calc(var(--wp-global-header-height) + var(--wp-admin--admin-bar--height));
+	z-index: 1;
+	display: flex;
+	justify-content: right;
+	padding: 1rem;
+	background: #fff;
+	border-width: 1px 1px 0;
+	border-style: solid;
+	border-color: #dcdcde;
+	border-top-left-radius: 0.3em;
+	border-top-right-radius: 0.3em;
+}
+
+@media screen and (max-width: 889px) {
+	.wp-code-block-button-container {
+		top: var(--wp-admin--admin-bar--height);
+	}
+}
+
+@media screen and (max-width: 600px) {
+	.wp-code-block-button-container {
+		top: 0;
+	}
+}
+
+.wp-code-block-button-container button {
+	font-size: 1.2rem !important;
+}
+
+.wp-code-block-button-container button + button {
+	margin-left: 0.5em;
+}
+
+.wp-code-block-button-container + pre {
+	margin-top: 0;
+	border-width: 0 1px 1px;
+	border-style: solid;
+	border-color: #dcdcde;
+	border-top-left-radius: 0;
+	border-top-right-radius: 0;
+}
diff --git a/mu-plugins/plugin-tweaks/index.php b/mu-plugins/plugin-tweaks/index.php
index fd0907d9..61291415 100644
--- a/mu-plugins/plugin-tweaks/index.php
+++ b/mu-plugins/plugin-tweaks/index.php
@@ -9,5 +9,6 @@
 
 defined( 'WPINC' ) || die();
 
-require_once __DIR__ . '/wporg-internal-notes.php';
+require_once __DIR__ . '/code-syntax-block/index.php';
 require_once __DIR__ . '/stream.php';
+require_once __DIR__ . '/wporg-internal-notes.php';