diff --git a/.github/workflows/phpunit.yml b/.github/workflows/phpunit.yml
index 44eb499..02cd580 100644
--- a/.github/workflows/phpunit.yml
+++ b/.github/workflows/phpunit.yml
@@ -60,3 +60,43 @@ jobs:
       - uses: ramsey/composer-install@a8d0d959dab41457692a5e2041bd9b757a119e3f # 3.2.1
 
       - run: composer test
+
+  coverage:
+    name: Coverage
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      actions: write
+    services:
+      mariadb:
+        image: mariadb:11
+        env:
+          MARIADB_ROOT_PASSWORD: root
+          MARIADB_DATABASE: wordpress_test
+        ports:
+          - 3306:3306
+        options: >-
+          --health-cmd="healthcheck.sh --connect --innodb_initialized"
+          --health-interval=5s
+          --health-timeout=5s
+          --health-retries=10
+    env:
+      WP_TESTS_DB_HOST: 127.0.0.1
+      WP_TESTS_DB_NAME: wordpress_test
+      WP_TESTS_DB_USER: root
+      WP_TESTS_DB_PASSWORD: root
+    steps:
+      - uses: actions/checkout@v5
+
+      - uses: shivammathur/setup-php@f3e473d116dcccaddc5834248c87452386958240 # 2.37.2
+        with:
+          php-version: '8.3'
+          extensions: mysqli
+          coverage: xdebug
+          tools: composer
+
+      - uses: ramsey/composer-install@a8d0d959dab41457692a5e2041bd9b757a119e3f # 3.2.1
+
+      - run: composer coverage
+
+      - uses: k1LoW/octocov-action@a167dc0dee441b7ffc45e1b862ab55ec0d87f278 # v1.5.2
diff --git a/.gitignore b/.gitignore
index 9851b06..10fdae2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 languages/*.mo
 /vendor/
 /vendor-bin/*/vendor/
+/tests/.coverage/
 /.phpunit.result.cache
 /node_modules/
 /tests/E2E/.results/
diff --git a/.octocov.yml b/.octocov.yml
new file mode 100644
index 0000000..dcb8e46
--- /dev/null
+++ b/.octocov.yml
@@ -0,0 +1,13 @@
+coverage:
+  paths:
+    - tests/.coverage/clover.xml
+  acceptable: current >= 90% && current >= prev
+
+diff:
+  datastores:
+    - artifact://${GITHUB_REPOSITORY}
+
+report:
+  if: is_default_branch
+  datastores:
+    - artifact://${GITHUB_REPOSITORY}
diff --git a/composer.json b/composer.json
index 44c3d60..9a29348 100644
--- a/composer.json
+++ b/composer.json
@@ -65,6 +65,10 @@
         ],
         "test:unit": "phpunit",
         "test:integration": "phpunit -c phpunit-integration.xml.dist",
+        "coverage": [
+            "@putenv XDEBUG_MODE=coverage",
+            "phpunit -c phpunit-coverage.xml.dist --coverage-clover=tests/.coverage/clover.xml"
+        ],
         "ci": [
             "@lint",
             "@cs",
diff --git a/phpunit-coverage.xml.dist b/phpunit-coverage.xml.dist
new file mode 100644
index 0000000..75ae9ca
--- /dev/null
+++ b/phpunit-coverage.xml.dist
@@ -0,0 +1,27 @@
+<?xml version="1.0"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor-bin/phpunit/vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="tests/bootstrap-integration.php"
+         colors="true"
+         failOnWarning="true"
+         failOnRisky="true"
+         beStrictAboutOutputDuringTests="true"
+         beStrictAboutTestsThatDoNotTestAnything="true"
+         convertDeprecationsToExceptions="false">
+    <testsuites>
+        <testsuite name="unit">
+            <directory suffix="Test.php">tests/Unit</directory>
+        </testsuite>
+        <testsuite name="integration">
+            <directory suffix="Test.php">tests/Integration</directory>
+        </testsuite>
+    </testsuites>
+    <coverage>
+        <include>
+            <file>functions.php</file>
+            <directory suffix=".php">inc</directory>
+            <directory suffix=".php">src</directory>
+            <directory suffix=".php">woocommerce</directory>
+        </include>
+    </coverage>
+</phpunit>
diff --git a/readme.txt b/readme.txt
index f8f5ba1..337b45d 100644
--- a/readme.txt
+++ b/readme.txt
@@ -58,6 +58,14 @@ Every check is a Composer script:
 - `composer stan`: PHPStan
 - `composer test`: PHPUnit
 - `composer ci`: all of the above, in this order
+- `composer coverage`: PHPUnit with a coverage report for octocov
+
+Every PHP file in `inc/`, `src/` and `woocommerce/` needs a test named after it,
+at the same path: under `tests/Integration` for `inc/` and `woocommerce/`, which
+need WordPress, and under `tests/Unit` or `tests/Integration` for `src/`. A browser
+test under `tests/E2E` follows the same names. `tests/Unit/StructureTest.php`
+enforces this and also fails on a test whose file no longer exists. In CI, octocov fails the run when line
+coverage is below 90% or below the last report of `main` (`.octocov.yml`).
 
 `composer install` brings in WordPress, the WordPress test suite and WooCommerce,
 so the tests only need a MySQL/MariaDB database they are allowed to wipe on every
diff --git a/tests/Support/ThemeFiles.php b/tests/Support/ThemeFiles.php
new file mode 100644
index 0000000..c7af470
--- /dev/null
+++ b/tests/Support/ThemeFiles.php
@@ -0,0 +1,35 @@
+<?php
+
+namespace LibreSign\WordPressTheme\Tests\Support;
+
+use FilesystemIterator;
+use RecursiveDirectoryIterator;
+use RecursiveIteratorIterator;
+
+final class ThemeFiles {
+
+	public static function root() {
+		return dirname( __DIR__, 2 );
+	}
+
+	public static function under( $directory, $suffix ) {
+		$root = self::root() . '/' . $directory;
+
+		if ( ! is_dir( $root ) ) {
+			return array();
+		}
+
+		$paths = array();
+		$files = new RecursiveIteratorIterator( new RecursiveDirectoryIterator( $root, FilesystemIterator::SKIP_DOTS ) );
+
+		foreach ( $files as $file ) {
+			if ( $file->isFile() && str_ends_with( $file->getPathname(), $suffix ) ) {
+				$paths[] = substr( $file->getPathname(), strlen( self::root() ) + 1 );
+			}
+		}
+
+		sort( $paths );
+
+		return $paths;
+	}
+}
diff --git a/tests/Unit/StructureTest.php b/tests/Unit/StructureTest.php
new file mode 100644
index 0000000..712c2d8
--- /dev/null
+++ b/tests/Unit/StructureTest.php
@@ -0,0 +1,103 @@
+<?php
+
+namespace LibreSign\WordPressTheme\Tests\Unit;
+
+use LibreSign\WordPressTheme\Tests\Support\ThemeFiles;
+use PHPUnit\Framework\TestCase;
+
+final class StructureTest extends TestCase {
+
+	/**
+	 * @dataProvider provide_theme_files
+	 */
+	public function test_a_file_of_the_theme_is_covered_by_the_test_named_after_it( $file ) {
+		$this->assert_one_exists( self::tests_covering( $file ), $file . ' is not covered by' );
+	}
+
+	public static function provide_theme_files() {
+		$files = array_merge(
+			ThemeFiles::under( 'inc', '.php' ),
+			ThemeFiles::under( 'src', '.php' ),
+			ThemeFiles::under( 'woocommerce', '.php' )
+		);
+
+		foreach ( $files as $file ) {
+			yield $file => array( $file );
+		}
+	}
+
+	/**
+	 * @dataProvider provide_test_files
+	 */
+	public function test_a_test_covers_a_file_of_the_theme( $file ) {
+		$this->assert_one_exists( self::files_covered_by( $file ), $file . ' does not cover' );
+	}
+
+	public static function provide_test_files() {
+		$files = array_merge(
+			ThemeFiles::under( 'tests/Unit', 'Test.php' ),
+			ThemeFiles::under( 'tests/Integration', 'Test.php' ),
+			ThemeFiles::under( 'tests/E2E', '.spec.ts' )
+		);
+
+		foreach ( $files as $file ) {
+			if ( 'tests/Unit/StructureTest.php' !== $file ) {
+				yield $file => array( $file );
+			}
+		}
+	}
+
+	private static function tests_covering( $file ) {
+		if ( str_starts_with( $file, 'src/' ) ) {
+			$name = substr( $file, strlen( 'src/' ), -strlen( '.php' ) );
+
+			return array(
+				'tests/Unit/' . $name . 'Test.php',
+				'tests/Integration/' . $name . 'Test.php',
+			);
+		}
+
+		return array( 'tests/Integration/' . self::test_path( substr( $file, 0, -strlen( '.php' ) ) ) . 'Test.php' );
+	}
+
+	private static function files_covered_by( $file ) {
+		if ( str_starts_with( $file, 'tests/E2E/' ) ) {
+			$name = substr( $file, strlen( 'tests/E2E/' ), -strlen( '.spec.ts' ) );
+
+			return array(
+				'src/' . $name . '.php',
+				self::theme_path( $name ) . '.php',
+			);
+		}
+
+		if ( str_starts_with( $file, 'tests/Unit/' ) ) {
+			return array( 'src/' . substr( $file, strlen( 'tests/Unit/' ), -strlen( 'Test.php' ) ) . '.php' );
+		}
+
+		$name = substr( $file, strlen( 'tests/Integration/' ), -strlen( 'Test.php' ) );
+
+		return array(
+			'src/' . $name . '.php',
+			self::theme_path( $name ) . '.php',
+		);
+	}
+
+	private function assert_one_exists( array $files, $subject ) {
+		$found = array_filter(
+			$files,
+			static function ( $file ) {
+				return file_exists( ThemeFiles::root() . '/' . $file );
+			}
+		);
+
+		$this->assertNotEmpty( $found, sprintf( '%s %s.', $subject, implode( ' or ', $files ) ) );
+	}
+
+	private static function test_path( $theme_path ) {
+		return implode( '/', array_map( static fn ( $part ) => str_replace( ' ', '', ucwords( str_replace( '-', ' ', $part ) ) ), explode( '/', $theme_path ) ) );
+	}
+
+	private static function theme_path( $test_path ) {
+		return implode( '/', array_map( static fn ( $part ) => strtolower( (string) preg_replace( '/(?<!^)[A-Z]/', '-$0', $part ) ), explode( '/', $test_path ) ) );
+	}
+}