diff --git a/.github/changelog/markpub-parser b/.github/changelog/markpub-parser
new file mode 100644
index 0000000..5beee3d
--- /dev/null
+++ b/.github/changelog/markpub-parser
@@ -0,0 +1,4 @@
+Significance: minor
+Type: added
+
+Add rich content support for standard.site documents using the Markpub format.
diff --git a/includes/class-atmosphere.php b/includes/class-atmosphere.php
index adef278..4a1a0fb 100644
--- a/includes/class-atmosphere.php
+++ b/includes/class-atmosphere.php
@@ -9,6 +9,7 @@
 
 \defined( 'ABSPATH' ) || exit;
 
+use Atmosphere\Content_Parser\Markpub;
 use Atmosphere\OAuth\Client;
 use Atmosphere\Transformer\Document;
 use Atmosphere\Transformer\Publication;
@@ -48,6 +49,9 @@ public function init(): void {
 		// Plugin integrations.
 		Load::init();
 
+		// Default content parser (Markpub).
+		\add_filter( 'atmosphere_content_parser', static fn() => new Markpub() );
+
 		// JSON preview for AT Protocol records.
 		\add_action( 'template_redirect', array( $this, 'preview' ) );
 
diff --git a/includes/content-parser/class-markpub.php b/includes/content-parser/class-markpub.php
new file mode 100644
index 0000000..a8e2005
--- /dev/null
+++ b/includes/content-parser/class-markpub.php
@@ -0,0 +1,435 @@
+<?php
+/**
+ * Markpub content parser.
+ *
+ * Converts WordPress block content into the at.markpub.markdown format
+ * by iterating Gutenberg blocks and converting each to its markdown
+ * equivalent.
+ *
+ * @see https://markpub.at/
+ *
+ * @package Atmosphere
+ */
+
+namespace Atmosphere\Content_Parser;
+
+\defined( 'ABSPATH' ) || exit;
+
+/**
+ * Markpub (at.markpub.markdown) content parser.
+ */
+class Markpub implements Content_Parser {
+
+	/**
+	 * {@inheritDoc}
+	 */
+	public function get_type(): string {
+		return 'at.markpub.markdown';
+	}
+
+	/**
+	 * {@inheritDoc}
+	 *
+	 * $post is required by the Content_Parser contract so parsers can
+	 * access post metadata; Markpub only needs $content.
+	 *
+	 * @param string   $content Raw post content.
+	 * @param \WP_Post $post    The WordPress post object.
+	 */
+	public function parse( string $content, \WP_Post $post ): ?array { // phpcs:ignore Generic.CodeAnalysis.UnusedFunctionParameter.Found, VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+		$blocks = \parse_blocks( $content );
+		$parts  = array();
+
+		foreach ( $blocks as $block ) {
+			$md = self::transform_block( $block );
+
+			if ( null !== $md ) {
+				$parts[] = $md;
+			}
+		}
+
+		$markdown = \implode( "\n\n", $parts );
+
+		/**
+		 * Filters the markdown produced from post content.
+		 *
+		 * @param string $markdown Converted markdown.
+		 * @param string $content  Raw post content.
+		 */
+		$markdown = \apply_filters( 'atmosphere_html_to_markdown', $markdown, $content );
+
+		if ( '' === \trim( $markdown ) ) {
+			return null;
+		}
+
+		return array(
+			'$type'      => 'at.markpub.markdown',
+			'text'       => array(
+				'$type'    => 'at.markpub.text',
+				'markdown' => $markdown,
+			),
+			'flavor'     => 'gfm',
+			'extensions' => array( 'strikethrough' ),
+		);
+	}
+
+	/**
+	 * Convert a single WordPress block to markdown.
+	 *
+	 * @param array $block Parsed block from parse_blocks().
+	 * @return string|null Markdown string or null to skip.
+	 */
+	private static function transform_block( array $block ): ?string {
+		if ( empty( $block['blockName'] ) ) {
+			// Classic (non-block) content or whitespace.
+			$md = self::inline_html_to_markdown( $block['innerHTML'] ?? '' );
+
+			return '' === $md ? null : $md;
+		}
+
+		return match ( $block['blockName'] ) {
+			'core/paragraph'    => self::paragraph( $block ),
+			'core/heading'      => self::heading( $block ),
+			'core/image'        => self::image( $block ),
+			'core/list'         => self::listing( $block ),
+			'core/quote'        => self::quote( $block ),
+			'core/code'         => self::code( $block ),
+			'core/preformatted' => self::preformatted( $block ),
+			'core/separator'    => '---',
+			'core/spacer'       => null,
+			'core/group',
+			'core/columns',
+			'core/column'       => self::container( $block ),
+			default             => self::fallback( $block ),
+		};
+	}
+
+	/**
+	 * Paragraph block.
+	 *
+	 * @param array $block Parsed block.
+	 * @return string|null
+	 */
+	private static function paragraph( array $block ): ?string {
+		$md = self::inline_html_to_markdown( $block['innerHTML'] ?? '' );
+
+		return '' === $md ? null : $md;
+	}
+
+	/**
+	 * Heading block.
+	 *
+	 * @param array $block Parsed block.
+	 * @return string|null
+	 */
+	private static function heading( array $block ): ?string {
+		$level = $block['attrs']['level'] ?? 2;
+		$text  = self::inline_html_to_markdown( $block['innerHTML'] ?? '' );
+
+		if ( empty( \trim( $text ) ) ) {
+			return null;
+		}
+
+		return \str_repeat( '#', (int) $level ) . ' ' . \trim( $text );
+	}
+
+	/**
+	 * Image block.
+	 *
+	 * @param array $block Parsed block.
+	 * @return string|null
+	 */
+	private static function image( array $block ): ?string {
+		$html = $block['innerHTML'] ?? '';
+		$src  = '';
+		$alt  = '';
+
+		$processor = new \WP_HTML_Tag_Processor( $html );
+		if ( $processor->next_tag( 'IMG' ) ) {
+			$src = $processor->get_attribute( 'src' ) ?? '';
+			$alt = $processor->get_attribute( 'alt' ) ?? '';
+		}
+
+		if ( empty( $src ) ) {
+			return null;
+		}
+
+		$md = '![' . $alt . '](' . $src . ')';
+
+		// Check for a caption in figcaption.
+		$caption_proc = new \WP_HTML_Tag_Processor( $html );
+		if ( $caption_proc->next_tag( 'FIGCAPTION' ) ) {
+			// Strip both ends of the figcaption tag BEFORE stripping
+			// remaining tags, so sibling content after </figcaption>
+			// (e.g. a trailing <p> inside the same <figure>) doesn't
+			// bleed into the caption text.
+			$caption = self::safe_replace( '#.*<figcaption[^>]*>#si', '', $html );
+			$caption = self::safe_replace( '#</figcaption>.*#si', '', $caption );
+			$caption = \trim( \wp_strip_all_tags( $caption ) );
+
+			if ( ! empty( $caption ) ) {
+				$md .= "\n" . $caption;
+			}
+		}
+
+		return $md;
+	}
+
+	/**
+	 * List block.
+	 *
+	 * @param array $block Parsed block.
+	 * @return string|null
+	 */
+	private static function listing( array $block ): ?string {
+		$ordered = ! empty( $block['attrs']['ordered'] );
+		$items   = array();
+		$counter = 0;
+
+		if ( ! empty( $block['innerBlocks'] ) ) {
+			foreach ( $block['innerBlocks'] as $inner ) {
+				$text = self::inline_html_to_markdown( $inner['innerHTML'] ?? '' );
+				$text = \trim( $text );
+
+				if ( empty( $text ) ) {
+					continue;
+				}
+
+				++$counter;
+				$prefix  = $ordered ? $counter . '. ' : '- ';
+				$items[] = $prefix . $text;
+			}
+		}
+
+		return empty( $items ) ? null : \implode( "\n", $items );
+	}
+
+	/**
+	 * Quote block.
+	 *
+	 * @param array $block Parsed block.
+	 * @return string|null
+	 */
+	private static function quote( array $block ): ?string {
+		$lines = array();
+
+		if ( ! empty( $block['innerBlocks'] ) ) {
+			foreach ( $block['innerBlocks'] as $inner ) {
+				$md = self::transform_block( $inner );
+				if ( null !== $md ) {
+					$lines[] = $md;
+				}
+			}
+		}
+
+		if ( empty( $lines ) ) {
+			$text = self::inline_html_to_markdown( $block['innerHTML'] ?? '' );
+			if ( empty( \trim( $text ) ) ) {
+				return null;
+			}
+			$lines = array( \trim( $text ) );
+		}
+
+		$quoted = \implode( "\n", $lines );
+
+		// Prefix each line with >.
+		return \implode(
+			"\n",
+			\array_map(
+				static fn( $line ) => '> ' . $line,
+				\explode( "\n", $quoted )
+			)
+		);
+	}
+
+	/**
+	 * Code block.
+	 *
+	 * @param array $block Parsed block.
+	 * @return string|null
+	 */
+	private static function code( array $block ): ?string {
+		$text = \wp_strip_all_tags( $block['innerHTML'] ?? '' );
+		$text = \html_entity_decode( $text, ENT_QUOTES, 'UTF-8' );
+		$text = \trim( $text );
+
+		if ( empty( $text ) ) {
+			return null;
+		}
+
+		$lang = $block['attrs']['language'] ?? '';
+
+		return '```' . $lang . "\n" . $text . "\n```";
+	}
+
+	/**
+	 * Preformatted block.
+	 *
+	 * @param array $block Parsed block.
+	 * @return string|null
+	 */
+	private static function preformatted( array $block ): ?string {
+		return self::code( $block );
+	}
+
+	/**
+	 * Container block — flatten inner blocks.
+	 *
+	 * @param array $block Parsed block.
+	 * @return string|null
+	 */
+	private static function container( array $block ): ?string {
+		if ( empty( $block['innerBlocks'] ) ) {
+			return null;
+		}
+
+		$parts = array();
+
+		foreach ( $block['innerBlocks'] as $inner ) {
+			$md = self::transform_block( $inner );
+			if ( null !== $md ) {
+				$parts[] = $md;
+			}
+		}
+
+		return empty( $parts ) ? null : \implode( "\n\n", $parts );
+	}
+
+	/**
+	 * Fallback for unknown block types.
+	 *
+	 * @param array $block Parsed block.
+	 * @return string|null
+	 */
+	private static function fallback( array $block ): ?string {
+		if ( ! empty( $block['innerBlocks'] ) ) {
+			return self::container( $block );
+		}
+
+		$md = self::inline_html_to_markdown( $block['innerHTML'] ?? '' );
+
+		return '' === $md ? null : $md;
+	}
+
+	/**
+	 * Convert inline HTML formatting to markdown.
+	 *
+	 * Handles links, bold, italic, strikethrough, inline code,
+	 * images, and line breaks. Strips block-level wrappers and
+	 * remaining tags.
+	 *
+	 * @param string $html HTML string.
+	 * @return string Markdown string.
+	 */
+	private static function inline_html_to_markdown( string $html ): string {
+		$html = \trim( $html );
+
+		if ( empty( $html ) ) {
+			return '';
+		}
+
+		$md = $html;
+
+		// Inline images.
+		$md = self::safe_replace_callback(
+			'#<img[^>]+>#si',
+			static function ( $m ) {
+				$processor = new \WP_HTML_Tag_Processor( $m[0] );
+				if ( $processor->next_tag( 'IMG' ) ) {
+					$src = $processor->get_attribute( 'src' ) ?? '';
+					$alt = $processor->get_attribute( 'alt' ) ?? '';
+					return '![' . $alt . '](' . $src . ')';
+				}
+				return '';
+			},
+			$md
+		);
+
+		// Links — percent-encode parentheses to avoid breaking markdown syntax.
+		$md = self::safe_replace_callback(
+			'#<a[^>]+href=["\']([^"\']*)["\'][^>]*>(.*?)</a>#si',
+			static fn( $m ) => '[' . \wp_strip_all_tags( $m[2] ) . '](' . \str_replace( array( '(', ')' ), array( '%28', '%29' ), $m[1] ) . ')',
+			$md
+		);
+
+		// Bold.
+		$md = self::safe_replace( '#<(?:strong|b)>(.*?)</(?:strong|b)>#si', '**$1**', $md );
+
+		// Italic.
+		$md = self::safe_replace( '#<(?:em|i)>(.*?)</(?:em|i)>#si', '*$1*', $md );
+
+		// Strikethrough.
+		$md = self::safe_replace( '#<(?:s|del|strike)>(.*?)</(?:s|del|strike)>#si', '~~$1~~', $md );
+
+		// Inline code.
+		$md = self::safe_replace( '#<code>(.*?)</code>#si', '`$1`', $md );
+
+		// Line breaks.
+		$md = self::safe_replace( '#<br\s*/?\s*>#si', "  \n", $md );
+
+		// Strip block-level wrappers and remaining tags.
+		$md = \wp_strip_all_tags( $md );
+
+		// Decode HTML entities.
+		$md = \html_entity_decode( $md, ENT_QUOTES, 'UTF-8' );
+
+		return \trim( $md );
+	}
+
+	/**
+	 * Wraps preg_replace with a fallback that preserves the input on PCRE failure.
+	 *
+	 * The underlying preg_replace returns null on engine failure
+	 * (e.g. backtrack or recursion limit hit on pathological input).
+	 * Without a guard, null cascades through subsequent string
+	 * operations and can erase the whole buffer with no signal.
+	 *
+	 * @param string $pattern     Pattern.
+	 * @param string $replacement Replacement.
+	 * @param string $subject     Input.
+	 * @return string Replaced string, or the original on PCRE failure.
+	 */
+	private static function safe_replace( string $pattern, string $replacement, string $subject ): string {
+		$result = \preg_replace( $pattern, $replacement, $subject );
+
+		if ( null === $result ) {
+			self::warn_pcre_failure( $pattern );
+			return $subject;
+		}
+
+		return $result;
+	}
+
+	/**
+	 * Wraps preg_replace_callback with the same failure guard as safe_replace().
+	 *
+	 * @param string   $pattern  Pattern.
+	 * @param callable $callback Callback.
+	 * @param string   $subject  Input.
+	 * @return string Replaced string, or the original on PCRE failure.
+	 */
+	private static function safe_replace_callback( string $pattern, callable $callback, string $subject ): string {
+		$result = \preg_replace_callback( $pattern, $callback, $subject );
+
+		if ( null === $result ) {
+			self::warn_pcre_failure( $pattern );
+			return $subject;
+		}
+
+		return $result;
+	}
+
+	/**
+	 * Emit a warning about a PCRE failure without hard-failing.
+	 *
+	 * @param string $pattern The pattern that failed.
+	 */
+	private static function warn_pcre_failure( string $pattern ): void {
+		if ( \function_exists( 'wp_trigger_error' ) ) {
+			\wp_trigger_error(
+				__METHOD__,
+				\sprintf( 'PCRE failure on pattern %s; preserving input.', $pattern )
+			);
+		}
+	}
+}
diff --git a/includes/content-parser/interface-content-parser.php b/includes/content-parser/interface-content-parser.php
index 4f11827..c9264b0 100644
--- a/includes/content-parser/interface-content-parser.php
+++ b/includes/content-parser/interface-content-parser.php
@@ -21,7 +21,10 @@ interface Content_Parser {
 	 * Parse WordPress post content into an AT Protocol content object.
 	 *
 	 * The returned array must include a '$type' key identifying the
-	 * lexicon type (e.g. 'at.markpub.markdown').
+	 * lexicon type (e.g. 'at.markpub.markdown'). Return null to signal
+	 * that the parser produced no usable output — Document will then
+	 * omit the content field — which is preferable to shipping an
+	 * empty-text record.
 	 *
 	 * Receives raw post content so parsers can choose their own
 	 * strategy: parse_blocks() for block-aware parsing, or
@@ -29,9 +32,9 @@ interface Content_Parser {
 	 *
 	 * @param string   $content Raw post content (post_content).
 	 * @param \WP_Post $post    The WordPress post object.
-	 * @return array AT Protocol content object.
+	 * @return array|null AT Protocol content object, or null to omit.
 	 */
-	public function parse( string $content, \WP_Post $post ): array;
+	public function parse( string $content, \WP_Post $post ): ?array;
 
 	/**
 	 * The lexicon NSID this parser produces.
diff --git a/includes/transformer/class-document.php b/includes/transformer/class-document.php
index 98a8a72..3745616 100644
--- a/includes/transformer/class-document.php
+++ b/includes/transformer/class-document.php
@@ -177,6 +177,10 @@ private function get_content(): ?array {
 
 		$content = $parser->parse( $this->object->post_content, $this->object );
 
+		if ( null === $content ) {
+			return null;
+		}
+
 		/**
 		 * Filters the parsed content object before adding to the document record.
 		 *
diff --git a/tests/phpunit/tests/content-parser/class-test-markpub.php b/tests/phpunit/tests/content-parser/class-test-markpub.php
new file mode 100644
index 0000000..bf59ea3
--- /dev/null
+++ b/tests/phpunit/tests/content-parser/class-test-markpub.php
@@ -0,0 +1,591 @@
+<?php
+/**
+ * Tests for the Markpub content parser.
+ *
+ * @package Atmosphere
+ * @group atmosphere
+ * @group content-parser
+ */
+
+namespace Atmosphere\Tests\Content_Parser;
+
+use WP_UnitTestCase;
+use Atmosphere\Content_Parser\Markpub;
+
+/**
+ * Markpub parser tests.
+ */
+class Test_Markpub extends WP_UnitTestCase {
+
+	/**
+	 * Parser instance.
+	 *
+	 * @var Markpub
+	 */
+	private Markpub $parser;
+
+	/**
+	 * Set up test fixtures.
+	 */
+	public function set_up(): void {
+		parent::set_up();
+		$this->parser = new Markpub();
+	}
+
+	/**
+	 * Test get_type returns the markpub NSID.
+	 */
+	public function test_get_type() {
+		$this->assertSame( 'at.markpub.markdown', $this->parser->get_type() );
+	}
+
+	/**
+	 * Test parse returns correct top-level structure.
+	 */
+	public function test_parse_returns_correct_structure() {
+		$post   = self::factory()->post->create_and_get();
+		$result = $this->parser->parse(
+			'<!-- wp:paragraph --><p>Hello world</p><!-- /wp:paragraph -->',
+			$post
+		);
+
+		$this->assertArrayHasKey( '$type', $result );
+		$this->assertSame( 'at.markpub.markdown', $result['$type'] );
+		$this->assertArrayHasKey( 'text', $result );
+		$this->assertSame( 'at.markpub.text', $result['text']['$type'] );
+		$this->assertArrayHasKey( 'markdown', $result['text'] );
+		$this->assertSame( 'gfm', $result['flavor'] );
+		$this->assertContains( 'strikethrough', $result['extensions'] );
+	}
+
+	/**
+	 * Test paragraph conversion.
+	 */
+	public function test_converts_paragraphs() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:paragraph -->\n<p>First paragraph</p>\n<!-- /wp:paragraph -->\n\n"
+			. "<!-- wp:paragraph -->\n<p>Second paragraph</p>\n<!-- /wp:paragraph -->";
+
+		$result   = $this->parser->parse( $content, $post );
+		$markdown = $result['text']['markdown'];
+
+		$this->assertStringContainsString( 'First paragraph', $markdown );
+		$this->assertStringContainsString( 'Second paragraph', $markdown );
+		$this->assertStringNotContainsString( '<p>', $markdown );
+	}
+
+	/**
+	 * Test heading conversion.
+	 */
+	public function test_converts_headings() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:heading {"level":2} --><h2>My Heading</h2><!-- /wp:heading -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertSame( '## My Heading', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test heading level 3.
+	 */
+	public function test_converts_heading_level_3() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:heading {"level":3} --><h3>Sub Heading</h3><!-- /wp:heading -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertSame( '### Sub Heading', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test link conversion in a paragraph.
+	 */
+	public function test_converts_links() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p>Visit <a href="https://example.com">Example</a> today.</p><!-- /wp:paragraph -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertStringContainsString( '[Example](https://example.com)', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test bold conversion.
+	 */
+	public function test_converts_bold() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p>This is <strong>bold</strong> text.</p><!-- /wp:paragraph -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertStringContainsString( '**bold**', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test italic conversion.
+	 */
+	public function test_converts_italic() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p>This is <em>italic</em> text.</p><!-- /wp:paragraph -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertStringContainsString( '*italic*', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test image block conversion.
+	 */
+	public function test_converts_images() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:image --><figure class="wp-block-image"><img src="https://example.com/photo.jpg" alt="A photo" /></figure><!-- /wp:image -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertStringContainsString( '![A photo](https://example.com/photo.jpg)', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test code block conversion.
+	 */
+	public function test_converts_code_blocks() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:code --><pre class="wp-block-code"><code>echo "hello";</code></pre><!-- /wp:code -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertSame( "```\necho \"hello\";\n```", $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test inline code conversion.
+	 */
+	public function test_converts_inline_code() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p>Use the <code>parse()</code> method.</p><!-- /wp:paragraph -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertStringContainsString( '`parse()`', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test separator block becomes horizontal rule.
+	 */
+	public function test_converts_separator() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:paragraph --><p>Before</p><!-- /wp:paragraph -->\n\n"
+			. "<!-- wp:separator --><hr class=\"wp-block-separator\"/><!-- /wp:separator -->\n\n"
+			. '<!-- wp:paragraph --><p>After</p><!-- /wp:paragraph -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertSame( "Before\n\n---\n\nAfter", $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test empty content returns null so Document can omit content.
+	 */
+	public function test_empty_content() {
+		$post = self::factory()->post->create_and_get();
+
+		$this->assertNull( $this->parser->parse( '', $post ) );
+	}
+
+	/**
+	 * Test the atmosphere_html_to_markdown filter.
+	 *
+	 * Verifies the filter callback receives ($markdown, $content) so
+	 * callers can inspect the raw source alongside the conversion.
+	 */
+	public function test_html_to_markdown_filter() {
+		$received = array();
+
+		\add_filter(
+			'atmosphere_html_to_markdown',
+			static function ( $markdown, $content ) use ( &$received ) {
+				$received = array(
+					'markdown' => $markdown,
+					'content'  => $content,
+				);
+				return 'custom markdown';
+			},
+			10,
+			2
+		);
+
+		$post   = self::factory()->post->create_and_get();
+		$source = '<!-- wp:paragraph --><p>Hello</p><!-- /wp:paragraph -->';
+		$result = $this->parser->parse( $source, $post );
+
+		$this->assertSame( 'custom markdown', $result['text']['markdown'] );
+		$this->assertSame( 'Hello', $received['markdown'] );
+		$this->assertSame( $source, $received['content'] );
+
+		\remove_all_filters( 'atmosphere_html_to_markdown' );
+	}
+
+	/**
+	 * Test strikethrough conversion.
+	 */
+	public function test_converts_strikethrough() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p>This is <del>deleted</del> text.</p><!-- /wp:paragraph -->';
+		$result  = $this->parser->parse( $content, $post );
+
+		$this->assertStringContainsString( '~~deleted~~', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test classic (non-block) content is handled as fallback.
+	 */
+	public function test_classic_content_fallback() {
+		$post   = self::factory()->post->create_and_get();
+		$result = $this->parser->parse( '<p>Classic editor content with <strong>bold</strong>.</p>', $post );
+		$md     = $result['text']['markdown'];
+
+		$this->assertStringContainsString( '**bold**', $md );
+		$this->assertStringContainsString( 'Classic editor content', $md );
+	}
+
+	/**
+	 * Test that sibling content after </figcaption> inside the same
+	 * <figure> does not bleed into the extracted caption text.
+	 */
+	public function test_image_caption_does_not_include_sibling_content() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:image -->\n"
+			. '<figure class="wp-block-image">'
+			. '<img src="https://example.com/photo.jpg" alt="A photo" />'
+			. '<figcaption>Real caption</figcaption>'
+			. '<p>Should not appear in caption</p>'
+			. '</figure>'
+			. "\n<!-- /wp:image -->";
+
+		$result = $this->parser->parse( $content, $post );
+		$md     = $result['text']['markdown'];
+
+		$this->assertStringContainsString( 'Real caption', $md );
+		$this->assertStringNotContainsString( 'Should not appear in caption', $md );
+	}
+
+	/**
+	 * Test that a post made up entirely of blocks that produce no
+	 * markdown (e.g. core/spacer) returns null so Document can omit
+	 * the content field.
+	 */
+	public function test_parse_returns_null_when_markdown_is_empty() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:spacer {\"height\":\"20px\"} -->\n"
+			. '<div style="height:20px" aria-hidden="true" class="wp-block-spacer"></div>' . "\n"
+			. '<!-- /wp:spacer -->';
+
+		$this->assertNull( $this->parser->parse( $content, $post ) );
+	}
+
+	/**
+	 * Test ordered list produces numbered markdown.
+	 */
+	public function test_listing_ordered() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:list {\"ordered\":true} -->\n<ol>"
+			. '<!-- wp:list-item --><li>First</li><!-- /wp:list-item -->'
+			. '<!-- wp:list-item --><li>Second</li><!-- /wp:list-item -->'
+			. '<!-- wp:list-item --><li>Third</li><!-- /wp:list-item -->'
+			. "</ol>\n<!-- /wp:list -->";
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( "1. First\n2. Second\n3. Third", $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test unordered list produces dashed markdown.
+	 */
+	public function test_listing_unordered() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:list -->\n<ul>"
+			. '<!-- wp:list-item --><li>First</li><!-- /wp:list-item -->'
+			. '<!-- wp:list-item --><li>Second</li><!-- /wp:list-item -->'
+			. '<!-- wp:list-item --><li>Third</li><!-- /wp:list-item -->'
+			. "</ul>\n<!-- /wp:list -->";
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( "- First\n- Second\n- Third", $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test ordered list skips empty items without gapping the counter.
+	 */
+	public function test_listing_skips_empty_items_without_gap() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:list {\"ordered\":true} -->\n<ol>"
+			. '<!-- wp:list-item --><li>First</li><!-- /wp:list-item -->'
+			. '<!-- wp:list-item --><li>   </li><!-- /wp:list-item -->'
+			. '<!-- wp:list-item --><li>Third</li><!-- /wp:list-item -->'
+			. "</ol>\n<!-- /wp:list -->";
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( "1. First\n2. Third", $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test list items preserve inline formatting.
+	 */
+	public function test_listing_preserves_inline_formatting() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:list -->\n<ul>"
+			. '<!-- wp:list-item --><li>some <strong>bold</strong></li><!-- /wp:list-item -->'
+			. "</ul>\n<!-- /wp:list -->";
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( '- some **bold**', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test quote block wraps an inner paragraph in a "> " prefix.
+	 */
+	public function test_quote_with_inner_paragraph() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:quote --><blockquote class="wp-block-quote">'
+			. '<!-- wp:paragraph --><p>Paragraph text</p><!-- /wp:paragraph -->'
+			. '</blockquote><!-- /wp:quote -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( '> Paragraph text', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test quote block prefixes every inner line.
+	 */
+	public function test_quote_prefixes_every_line() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:quote --><blockquote class="wp-block-quote">'
+			. '<!-- wp:paragraph --><p>First</p><!-- /wp:paragraph -->'
+			. '<!-- wp:paragraph --><p>Second</p><!-- /wp:paragraph -->'
+			. '</blockquote><!-- /wp:quote -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( "> First\n> Second", $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test quote falls back to innerHTML when no innerBlocks are present.
+	 */
+	public function test_quote_innerhtml_fallback() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:quote --><blockquote class="wp-block-quote">Direct quote text</blockquote><!-- /wp:quote -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( '> Direct quote text', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test core/group containers flatten inner block markdown.
+	 */
+	public function test_container_group() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:group --><div class="wp-block-group">'
+			. '<!-- wp:paragraph --><p>Inside group</p><!-- /wp:paragraph -->'
+			. '</div><!-- /wp:group -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( 'Inside group', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test core/columns containers flatten inner block markdown.
+	 */
+	public function test_container_columns() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:columns --><div class="wp-block-columns">'
+			. '<!-- wp:paragraph --><p>Inside columns</p><!-- /wp:paragraph -->'
+			. '</div><!-- /wp:columns -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( 'Inside columns', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test core/column containers flatten inner block markdown.
+	 */
+	public function test_container_column() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:column --><div class="wp-block-column">'
+			. '<!-- wp:paragraph --><p>Inside column</p><!-- /wp:paragraph -->'
+			. '</div><!-- /wp:column -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( 'Inside column', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test fallback delegates to container() when innerBlocks exist.
+	 */
+	public function test_fallback_delegates_to_container_with_inner_blocks() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:custom/unknown --><div>'
+			. '<!-- wp:paragraph --><p>Inside unknown</p><!-- /wp:paragraph -->'
+			. '</div><!-- /wp:custom/unknown -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( 'Inside unknown', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test image() skips blocks without an <img> tag so surrounding
+	 * content renders with no empty separator.
+	 *
+	 * Uses a mixed fixture so a regression returning "" instead of null
+	 * would produce a leading blank line and fail this exact-match
+	 * assertion (the whole-post empty guard in parse() would otherwise
+	 * mask the handler bug).
+	 */
+	public function test_image_without_img_tag_is_skipped_cleanly() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:image --><figure class="wp-block-image">'
+			. '<figcaption>Just a caption</figcaption>'
+			. "</figure><!-- /wp:image -->\n\n"
+			. '<!-- wp:paragraph --><p>After</p><!-- /wp:paragraph -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( 'After', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test heading defaults to level 2 when attrs.level is missing.
+	 */
+	public function test_heading_defaults_to_level_2() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:heading --><h2>Default level</h2><!-- /wp:heading -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( '## Default level', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test whitespace-only heading block is skipped cleanly.
+	 *
+	 * Mixed with a non-empty sibling so a regression returning "" from
+	 * heading() would produce a leading blank line and fail the exact
+	 * assertion (the whole-post empty guard would otherwise hide it).
+	 */
+	public function test_heading_whitespace_is_skipped_cleanly() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:heading --><h2>   </h2><!-- /wp:heading -->\n\n"
+			. '<!-- wp:paragraph --><p>After</p><!-- /wp:paragraph -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( 'After', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test whitespace-only paragraph block is skipped cleanly.
+	 *
+	 * Mixed with a non-empty sibling so a regression returning "" from
+	 * paragraph() would produce a leading blank line and fail the exact
+	 * assertion (the whole-post empty guard would otherwise hide it).
+	 */
+	public function test_paragraph_whitespace_is_skipped_cleanly() {
+		$post    = self::factory()->post->create_and_get();
+		$content = "<!-- wp:paragraph --><p>   </p><!-- /wp:paragraph -->\n\n"
+			. '<!-- wp:paragraph --><p>After</p><!-- /wp:paragraph -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( 'After', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test code block emits the configured language in the fence.
+	 */
+	public function test_code_emits_language_fence() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:code {"language":"php"} --><pre class="wp-block-code"><code>echo 1;</code></pre><!-- /wp:code -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertStringStartsWith( "```php\n", $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test code block decodes HTML entities inside the fence.
+	 */
+	public function test_code_decodes_html_entities() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:code --><pre class="wp-block-code"><code>&lt;div&gt;</code></pre><!-- /wp:code -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( "```\n<div>\n```", $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test link URLs have parentheses percent-encoded to protect markdown syntax.
+	 */
+	public function test_link_url_parens_percent_encoded() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p>See <a href="https://en.wikipedia.org/wiki/Foo_(bar)">Foo</a>.</p><!-- /wp:paragraph -->';
+
+		$result = $this->parser->parse( $content, $post );
+		$md     = $result['text']['markdown'];
+
+		$this->assertStringContainsString( '%28bar%29', $md );
+		$this->assertStringNotContainsString( '(bar)', $md );
+	}
+
+	/**
+	 * Test <br> converts to a markdown hard break (two spaces + newline).
+	 */
+	public function test_br_converts_to_hard_break() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p>line1<br>line2</p><!-- /wp:paragraph -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertStringContainsString( "line1  \nline2", $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test HTML entities are decoded in inline paragraph text.
+	 */
+	public function test_inline_html_entities_decoded() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p>AT&amp;T&#8217;s</p><!-- /wp:paragraph -->';
+
+		$result = $this->parser->parse( $content, $post );
+		$md     = $result['text']['markdown'];
+
+		$this->assertStringContainsString( 'AT&T', $md );
+		$this->assertStringContainsString( "\xE2\x80\x99", $md );
+	}
+
+	/**
+	 * Test inline <img> inside a paragraph converts via inline_html_to_markdown.
+	 */
+	public function test_inline_image_inside_paragraph() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p>Look <img src="x.jpg" alt="x"> here</p><!-- /wp:paragraph -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( 'Look ![x](x.jpg) here', $result['text']['markdown'] );
+	}
+
+	/**
+	 * Test nested inline formatting (bold wrapping italic).
+	 */
+	public function test_nested_inline_formatting() {
+		$post    = self::factory()->post->create_and_get();
+		$content = '<!-- wp:paragraph --><p><strong>bold <em>italic</em></strong></p><!-- /wp:paragraph -->';
+
+		$result = $this->parser->parse( $content, $post );
+
+		$this->assertSame( '**bold *italic***', $result['text']['markdown'] );
+	}
+}
diff --git a/tests/phpunit/tests/transformer/class-stub-parser.php b/tests/phpunit/tests/transformer/class-stub-parser.php
index b869820..0231dd8 100644
--- a/tests/phpunit/tests/transformer/class-stub-parser.php
+++ b/tests/phpunit/tests/transformer/class-stub-parser.php
@@ -14,6 +14,13 @@
  */
 class Stub_Parser implements Content_Parser {
 
+	/**
+	 * Whether parse() should return null.
+	 *
+	 * @var bool
+	 */
+	public bool $return_null = false;
+
 	/**
 	 * {@inheritDoc}
 	 */
@@ -27,7 +34,11 @@ public function get_type(): string {
 	 * @param string   $content Raw post content.
 	 * @param \WP_Post $post    The WordPress post object.
 	 */
-	public function parse( string $content, \WP_Post $post ): array { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+	public function parse( string $content, \WP_Post $post ): ?array { // phpcs:ignore Generic.CodeAnalysis.UnusedFunctionParameter.Found, VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+		if ( $this->return_null ) {
+			return null;
+		}
+
 		return array(
 			'$type' => 'test.stub.parser',
 			'text'  => $content,
diff --git a/tests/phpunit/tests/transformer/class-test-document.php b/tests/phpunit/tests/transformer/class-test-document.php
index 3915ac3..01c24a1 100644
--- a/tests/phpunit/tests/transformer/class-test-document.php
+++ b/tests/phpunit/tests/transformer/class-test-document.php
@@ -20,9 +20,12 @@
 class Test_Document extends WP_UnitTestCase {
 
 	/**
-	 * Test that content field is absent when no parser is registered.
+	 * Test that content field is absent when parser filter returns null.
 	 */
 	public function test_content_absent_without_parser() {
+		\remove_all_filters( 'atmosphere_content_parser' );
+		\add_filter( 'atmosphere_content_parser', '__return_null' );
+
 		$post = self::factory()->post->create_and_get(
 			array( 'post_content' => 'Some content here.' )
 		);
@@ -31,6 +34,8 @@ public function test_content_absent_without_parser() {
 		$record      = $transformer->transform();
 
 		$this->assertArrayNotHasKey( 'content', $record );
+
+		\remove_all_filters( 'atmosphere_content_parser' );
 	}
 
 	/**
@@ -95,6 +100,40 @@ public function test_content_ignored_with_invalid_parser() {
 		\remove_all_filters( 'atmosphere_content_parser' );
 	}
 
+	/**
+	 * Test that when the parser returns null for non-empty content,
+	 * the content field is omitted and the atmosphere_document_content
+	 * filter is not invoked.
+	 */
+	public function test_content_absent_when_parser_returns_null() {
+		$parser              = new Stub_Parser();
+		$parser->return_null = true;
+
+		\add_filter( 'atmosphere_content_parser', static fn() => $parser );
+
+		$filter_called = false;
+		\add_filter(
+			'atmosphere_document_content',
+			static function ( $content ) use ( &$filter_called ) {
+				$filter_called = true;
+				return $content;
+			}
+		);
+
+		$post = self::factory()->post->create_and_get(
+			array( 'post_content' => 'Some content.' )
+		);
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		$this->assertArrayNotHasKey( 'content', $record );
+		$this->assertFalse( $filter_called );
+
+		\remove_all_filters( 'atmosphere_content_parser' );
+		\remove_all_filters( 'atmosphere_document_content' );
+	}
+
 	/**
 	 * Test that content field is absent for empty post content.
 	 */