Fix display of images in Twenty Twenty to be block not inline-block

includes/sanitizers/class-amp-core-theme-sanitizer.php

@@ -94,6 +94,7 @@ protected static function get_theme_features_config( $theme_slug ) {
 					'add_twentytwenty_toggles'         => [],
 					'add_nav_menu_styles'              => [],
 					'add_twentytwenty_masthead_styles' => [],
+					'add_img_display_block_fix' => [],
 					'add_twentytwenty_current_page_awareness' => [],
 				];
 
@@ -719,6 +720,29 @@ static function() {
 		);
 	}
 
+	/**
+	 * Add style rule with a selector of higher specificity than just `img` to make `amp-img` have `display:block` rather than `display:inline-block`.
+	 *
+	 * This is needed to override the AMP core stylesheet which has a more specific selector `.i-amphtml-layout-intrinsic` which
+	 * is given a `display: line-block`; this display value prevents margins from collapsing with surrounding block elements,
+	 * resulting in larger margins in AMP than expected.
+	 *
+	 * @since 1.5
+	 */
+	public static function add_img_display_block_fix() {
+		add_action(
+			'wp_enqueue_scripts',
+			static function() {
+				wp_add_inline_style(
+					get_template() . '-style',
+					// The selector is targeting an attribute that can never appear. It is purely present to increase specificity.
+					'amp-img:not([_]) { display: block }'
+				);
+			},
+			11
+		);
+	}
+
 	/**
 	 * Add required styles for featured image header in Twenty Nineteen.
 	 *

tests/php/test-class-amp-core-theme-sanitizer.php

@@ -238,4 +238,19 @@ public function test_guess_modal_role( DOMElement $dom_element, $expected ) {
 
 		$this->assertEquals( $expected, $actual );
 	}
+
+	/**
+	 * Tests add_img_display_block_fix.
+	 *
+	 * @covers AMP_Core_Theme_Sanitizer::add_img_display_block_fix()
+	 */
+	public function test_add_img_display_block_fix() {
+		$handle = get_template() . '-style';
+		wp_register_style( $handle, get_stylesheet_uri(), [], '0.1' );
+
+		$this->assertEmpty( wp_styles()->print_inline_style( $handle, false ) );
+		AMP_Core_Theme_Sanitizer::add_img_display_block_fix();
+		wp_enqueue_scripts();
+		$this->assertRegExp( '/amp-img.+display.+block/s', wp_styles()->print_inline_style( $handle, false ) );
+	}
 }