diff --git a/src/wp-includes/class-wp-theme-json.php b/src/wp-includes/class-wp-theme-json.php
index 635f670cff7a7..1e1b5d02f9f7b 100644
--- a/src/wp-includes/class-wp-theme-json.php
+++ b/src/wp-includes/class-wp-theme-json.php
@@ -345,9 +345,9 @@ class WP_Theme_JSON {
 	 *
 	 * @since 5.8.0
 	 *
-	 * @param array $theme_json A structure that follows the theme.json schema.
-	 * @param string $origin    Optional. What source of data this object represents.
-	 *                          One of 'default', 'theme', or 'custom'. Default 'theme'.
+	 * @param array  $theme_json A structure that follows the theme.json schema.
+	 * @param string $origin     Optional. What source of data this object represents.
+	 *                           One of 'default', 'theme', or 'custom'. Default 'theme'.
 	 */
 	public function __construct( $theme_json = array(), $origin = 'theme' ) {
 		if ( ! in_array( $origin, self::VALID_ORIGINS, true ) ) {
@@ -764,6 +764,18 @@ private function get_block_classes( $style_nodes ) {
 				}
 			}
 
+			/*
+			 * Reset default browser margin on the root body element.
+			 * This is set on the root selector **before** generating the ruleset
+			 * from the `theme.json`. This is to ensure that if the `theme.json` declares
+			 * `margin` in its `spacing` declaration for the `body` element then these
+			 * user-generated values take precedence in the CSS cascade.
+			 * @link https://github.com/WordPress/gutenberg/issues/36147.
+			 */
+			if ( self::ROOT_BLOCK_SELECTOR === $selector ) {
+				$block_rules .= 'body { margin: 0; }';
+			}
+
 			// 2. Generate the rules that use the general selector.
 			$block_rules .= self::to_ruleset( $selector, $declarations );
 
@@ -774,7 +786,6 @@ private function get_block_classes( $style_nodes ) {
 			}
 
 			if ( self::ROOT_BLOCK_SELECTOR === $selector ) {
-				$block_rules .= 'body { margin: 0; }';
 				$block_rules .= '.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }';
 				$block_rules .= '.wp-site-blocks > .alignright { float: right; margin-left: 2em; }';
 				$block_rules .= '.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }';
diff --git a/tests/phpunit/tests/theme/wpThemeJson.php b/tests/phpunit/tests/theme/wpThemeJson.php
index 23f2505140b80..60c6d16eded1c 100644
--- a/tests/phpunit/tests/theme/wpThemeJson.php
+++ b/tests/phpunit/tests/theme/wpThemeJson.php
@@ -376,9 +376,9 @@ public function test_get_stylesheet_renders_enabled_protected_properties() {
 			)
 		);
 
-		$expected = 'body{--wp--style--block-gap: 1em;}body { margin: 0; }.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }.wp-site-blocks > .alignright { float: right; margin-left: 2em; }.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }.wp-site-blocks > * { margin-top: 0; margin-bottom: 0; }.wp-site-blocks > * + * { margin-top: var( --wp--style--block-gap ); }.wp-block-columns{--wp--style--block-gap: 24px;}';
-		$this->assertEquals( $expected, $theme_json->get_stylesheet() );
-		$this->assertEquals( $expected, $theme_json->get_stylesheet( array( 'styles' ) ) );
+		$expected = 'body { margin: 0; }body{--wp--style--block-gap: 1em;}.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }.wp-site-blocks > .alignright { float: right; margin-left: 2em; }.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }.wp-site-blocks > * { margin-top: 0; margin-bottom: 0; }.wp-site-blocks > * + * { margin-top: var( --wp--style--block-gap ); }.wp-block-columns{--wp--style--block-gap: 24px;}';
+		$this->assertSame( $expected, $theme_json->get_stylesheet() );
+		$this->assertSame( $expected, $theme_json->get_stylesheet( array( 'styles' ) ) );
 	}
 
 	/**
@@ -506,13 +506,13 @@ public function test_get_stylesheet() {
 		);
 
 		$variables = 'body{--wp--preset--color--grey: grey;--wp--preset--font-family--small: 14px;--wp--preset--font-family--big: 41px;}.wp-block-group{--wp--custom--base-font: 16;--wp--custom--line-height--small: 1.2;--wp--custom--line-height--medium: 1.4;--wp--custom--line-height--large: 1.8;}';
-		$styles    = 'body{color: var(--wp--preset--color--grey);}body { margin: 0; }.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }.wp-site-blocks > .alignright { float: right; margin-left: 2em; }.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }.wp-site-blocks > * { margin-top: 0; margin-bottom: 0; }.wp-site-blocks > * + * { margin-top: var( --wp--style--block-gap ); }a{background-color: #333;color: #111;}.wp-block-group{border-radius: 10px;padding: 24px;}.wp-block-group a{color: #111;}h1,h2,h3,h4,h5,h6{color: #123456;}h1 a,h2 a,h3 a,h4 a,h5 a,h6 a{background-color: #333;color: #111;font-size: 60px;}.wp-block-post-date{color: #123456;}.wp-block-post-date a{background-color: #777;color: #555;}.wp-block-image{border-top-left-radius: 10px;border-bottom-right-radius: 1em;margin-bottom: 30px;}';
+		$styles    = 'body { margin: 0; }body{color: var(--wp--preset--color--grey);}.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }.wp-site-blocks > .alignright { float: right; margin-left: 2em; }.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }.wp-site-blocks > * { margin-top: 0; margin-bottom: 0; }.wp-site-blocks > * + * { margin-top: var( --wp--style--block-gap ); }a{background-color: #333;color: #111;}.wp-block-group{border-radius: 10px;padding: 24px;}.wp-block-group a{color: #111;}h1,h2,h3,h4,h5,h6{color: #123456;}h1 a,h2 a,h3 a,h4 a,h5 a,h6 a{background-color: #333;color: #111;font-size: 60px;}.wp-block-post-date{color: #123456;}.wp-block-post-date a{background-color: #777;color: #555;}.wp-block-image{border-top-left-radius: 10px;border-bottom-right-radius: 1em;margin-bottom: 30px;}';
 		$presets   = '.has-grey-color{color: var(--wp--preset--color--grey) !important;}.has-grey-background-color{background-color: var(--wp--preset--color--grey) !important;}.has-grey-border-color{border-color: var(--wp--preset--color--grey) !important;}.has-small-font-family{font-family: var(--wp--preset--font-family--small) !important;}.has-big-font-family{font-family: var(--wp--preset--font-family--big) !important;}';
 		$all       = $variables . $styles . $presets;
-		$this->assertEquals( $all, $theme_json->get_stylesheet() );
-		$this->assertEquals( $styles, $theme_json->get_stylesheet( array( 'styles' ) ) );
-		$this->assertEquals( $presets, $theme_json->get_stylesheet( array( 'presets' ) ) );
-		$this->assertEquals( $variables, $theme_json->get_stylesheet( array( 'variables' ) ) );
+		$this->assertSame( $all, $theme_json->get_stylesheet() );
+		$this->assertSame( $styles, $theme_json->get_stylesheet( array( 'styles' ) ) );
+		$this->assertSame( $presets, $theme_json->get_stylesheet( array( 'presets' ) ) );
+		$this->assertSame( $variables, $theme_json->get_stylesheet( array( 'variables' ) ) );
 	}
 
 	/**