Try server-side rendering the block

# Conflicts:
#	packages/block-library/src/table-of-contents/save.js

lib/blocks.php

@@ -120,6 +120,7 @@ function gutenberg_reregister_core_block_types() {
 				'site-logo.php'                    => 'core/site-logo',
 				'site-tagline.php'                 => 'core/site-tagline',
 				'site-title.php'                   => 'core/site-title',
+				'table-of-contents.php'            => 'core/table-of-contents',
 				'tag-cloud.php'                    => 'core/tag-cloud',
 				'template-part.php'                => 'core/template-part',
 				'term-description.php'             => 'core/term-description',

packages/block-library/src/table-of-contents/index.js

@@ -10,7 +10,6 @@ import { tableOfContents as icon } from '@wordpress/icons';
 import initBlock from '../utils/init-block';
 import metadata from './block.json';
 import edit from './edit';
-import save from './save';
 
 const { name } = metadata;
 
@@ -19,7 +18,6 @@ export { metadata, name };
 export const settings = {
 	icon,
 	edit,
-	save,
 	example: {
 		innerBlocks: [
 			{

packages/block-library/src/table-of-contents/index.php

@@ -0,0 +1,135 @@
+<?php
+/**
+ * Server-side rendering of the `core/table-of-contents` block.
+ *
+ * @package WordPress
+ */
+
+/**
+ * Renders the `core/table-of-contents` block on the server.
+ *
+ * @param array    $attributes Block attributes.
+ * @return string Returns the table of contents markup.
+ */
+function render_block_core_table_of_contents( $attributes ) {
+	$headings = isset( $attributes['headings'] ) ? $attributes['headings'] : array();
+
+	if ( empty( $headings ) ) {
+		return '';
+	}
+
+	// Get the aria-label from block attributes, or fallback to localized default.
+	$aria_label = '';
+	if ( isset( $attributes['ariaLabel'] ) && ! empty( $attributes['ariaLabel'] ) ) {
+		$aria_label = $attributes['ariaLabel'];
+	} else {
+		$aria_label = __( 'Table of Contents', 'gutenberg' );
+	}
+
+	$wrapper_attributes = get_block_wrapper_attributes(
+		array(
+			'aria-label' => $aria_label,
+		)
+	);
+
+	$nested_headings = convert_linear_to_nested_headings( $headings );
+	$list_items      = build_nested_list_items( $nested_headings );
+
+	$html = sprintf(
+		'<nav %s><ol>%s</ol></nav>',
+		$wrapper_attributes,
+		$list_items
+	);
+
+	$processor = new WP_HTML_Tag_Processor( $html );
+	if ( $processor->next_tag( 'nav' ) ) {
+		$processor->set_attribute( 'aria-label', $aria_label );
+	}
+
+	return $processor->get_updated_html();
+}
+
+/**
+ * Converts linear heading list to nested structure.
+ *
+ * @param array $headings Linear array of headings.
+ * @return array Nested array of headings.
+ */
+function convert_linear_to_nested_headings( $headings ) {
+	$nested_heading_list = array();
+
+	foreach ( $headings as $key => $heading ) {
+		if ( empty( $heading['content'] ) ) {
+			continue;
+		}
+
+		if ( $heading['level'] === $headings[0]['level'] ) {
+			if ( isset( $headings[ $key + 1 ] ) && $headings[ $key + 1 ]['level'] > $heading['level'] ) {
+				$end_of_slice = count( $headings );
+				for ( $i = $key + 1; $i < count( $headings ); $i++ ) {
+					if ( $headings[ $i ]['level'] === $heading['level'] ) {
+						$end_of_slice = $i;
+						break;
+					}
+				}
+
+				$nested_heading_list[] = array(
+					'heading' => $heading,
+					'children' => convert_linear_to_nested_headings(
+						array_slice( $headings, $key + 1, $end_of_slice - $key - 1 )
+					),
+				);
+			} else {
+				$nested_heading_list[] = array(
+					'heading' => $heading,
+					'children' => array(),
+				);
+			}
+		}
+	}
+
+	return $nested_heading_list;
+}
+
+/**
+ * Builds nested list items HTML recursively.
+ *
+ * @param array $nested_headings Nested array of headings.
+ * @return string HTML for nested list items.
+ */
+function build_nested_list_items( $nested_headings ) {
+	$html = '';
+
+	foreach ( $nested_headings as $item ) {
+		$heading = $item['heading'];
+		$link = ! empty( $heading['link'] ) ? $heading['link'] : '';
+
+		$html .= '<li>';
+		$html .= sprintf(
+			'<a class="wp-block-table-of-contents__entry" href="%s">%s</a>',
+			esc_url( $link ),
+			esc_html( $heading['content'] )
+		);
+
+		if ( ! empty( $item['children'] ) ) {
+			$html .= '<ol>' . build_nested_list_items( $item['children'] ) . '</ol>';
+		}
+
+		$html .= '</li>';
+	}
+
+	return $html;
+}
+
+/**
+ * Registers the `core/table-of-contents` block on the server.
+ */
+function register_block_core_table_of_contents() {
+	register_block_type_from_metadata(
+		__DIR__ . '/table-of-contents',
+		array(
+			'render_callback' => 'render_block_core_table_of_contents',
+		)
+	);
+}
+add_action( 'init', 'register_block_core_table_of_contents' );

test/integration/fixtures/blocks/core__table-of-contents.html

@@ -1,3 +1 @@
-<!-- wp:table-of-contents {"headings":[{"content":"Heading text","level":2,"link":"#heading-id-1"},{"content":"A sub-heading","level":3,"link":"#heading-id-2"}]} -->
-<nav class="wp-block-table-of-contents" aria-label="Table of Contents"><ol><li><a class="wp-block-table-of-contents__entry" href="#heading-id-1">Heading text</a><ol><li><a class="wp-block-table-of-contents__entry" href="#heading-id-2">A sub-heading</a></li></ol></li></ol></nav>
-<!-- /wp:table-of-contents -->
+<!-- wp:table-of-contents {"headings":[{"content":"Heading text","level":2,"link":"#heading-id-1"},{"content":"A sub-heading","level":3,"link":"#heading-id-2"}]} /-->

test/integration/fixtures/blocks/core__table-of-contents.parsed.json

@@ -16,9 +16,7 @@
 			]
 		},
 		"innerBlocks": [],
-		"innerHTML": "\n<nav class=\"wp-block-table-of-contents\" aria-label=\"Table of Contents\"><ol><li><a class=\"wp-block-table-of-contents__entry\" href=\"#heading-id-1\">Heading text</a><ol><li><a class=\"wp-block-table-of-contents__entry\" href=\"#heading-id-2\">A sub-heading</a></li></ol></li></ol></nav>\n",
-		"innerContent": [
-			"\n<nav class=\"wp-block-table-of-contents\" aria-label=\"Table of Contents\"><ol><li><a class=\"wp-block-table-of-contents__entry\" href=\"#heading-id-1\">Heading text</a><ol><li><a class=\"wp-block-table-of-contents__entry\" href=\"#heading-id-2\">A sub-heading</a></li></ol></li></ol></nav>\n"
-		]
+		"innerHTML": "",
+		"innerContent": []
 	}
 ]

test/integration/fixtures/blocks/core__table-of-contents.serialized.html

@@ -1,3 +1 @@
-<!-- wp:table-of-contents {"headings":[{"content":"Heading text","level":2,"link":"#heading-id-1"},{"content":"A sub-heading","level":3,"link":"#heading-id-2"}]} -->
-<nav class="wp-block-table-of-contents" aria-label="Table of Contents"><ol><li><a class="wp-block-table-of-contents__entry" href="#heading-id-1">Heading text</a><ol><li><a class="wp-block-table-of-contents__entry" href="#heading-id-2">A sub-heading</a></li></ol></li></ol></nav>
-<!-- /wp:table-of-contents -->
+<!-- wp:table-of-contents {"headings":[{"content":"Heading text","level":2,"link":"#heading-id-1"},{"content":"A sub-heading","level":3,"link":"#heading-id-2"}]} /-->