diff --git a/.gitattributes b/.gitattributes
index c0c0e12f..c26292bd 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -8,4 +8,5 @@
 /CONTRIBUTING.md export-ignore
 /bin/ export-ignore
 /docs/ export-ignore
+/src/PhpStan/ export-ignore
 /tests/ export-ignore
diff --git a/Build/phpstan/phpstan.neon b/Build/phpstan/phpstan.neon
index 46e8153d..a21758dd 100644
--- a/Build/phpstan/phpstan.neon
+++ b/Build/phpstan/phpstan.neon
@@ -11,7 +11,8 @@ parameters:
     - %currentWorkingDirectory%/src/
     - %currentWorkingDirectory%/tests/
 
-  treatPhpDocTypesAsCertain: false
+  excludePaths:
+    - %currentWorkingDirectory%/tests/fixtures/phpstan/
 
   type_perfect:
     no_mixed_property: true
@@ -24,3 +25,9 @@ parameters:
     -
       message: '#^Call to static method PHPUnit\\Framework\\Assert\:\:assertInstanceOf\(\) .* will always evaluate to#'
       path: '%currentWorkingDirectory%/tests/'
+
+services:
+  -
+    class: \Sabberworm\CSS\PhpStan\IgnoreBooleanAlways
+    tags:
+      - phpstan.ignoreErrorExtension
diff --git a/src/PhpStan/IgnoreBooleanAlways.php b/src/PhpStan/IgnoreBooleanAlways.php
new file mode 100644
index 00000000..dbcd499f
--- /dev/null
+++ b/src/PhpStan/IgnoreBooleanAlways.php
@@ -0,0 +1,48 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\PhpStan;
+
+use PhpParser\Node;
+use PhpParser\Node\Expr\FuncCall;
+use PhpParser\Node\Name;
+use PHPStan\Analyser\Error;
+use PHPStan\Analyser\IgnoreErrorExtension;
+use PHPStan\Analyser\Scope;
+
+/**
+ * Ignore PHPStan warnings where the DocBlocks indicate that a conditional expression would always be true (or false),
+ * but a programming mistake elsewhere could lead to that not being the case, for the following:
+ * - `assert($object instanceof Class);`.
+ *
+ * @internal
+ */
+final class IgnoreBooleanAlways implements IgnoreErrorExtension
+{
+    public function shouldIgnore(Error $error, Node $node, Scope $scope): bool
+    {
+        $shouldIgnore = false;
+
+        switch ($error->getIdentifier()) {
+            case 'function.alreadyNarrowedType':
+                // For an `assert()` that the DocBlocks say cannot fail.
+                if ($node instanceof FuncCall) {
+                    $nameNode = $node->name;
+                    if ($nameNode instanceof Name && $nameNode->name === 'assert') {
+                        $shouldIgnore = true;
+                    }
+                }
+                break;
+            case 'instanceof.alwaysTrue':
+                // For `instanceof` within an `assert()` that the DocBlocks say cannot fail.
+                $functionCallStack = $scope->getFunctionCallStack();
+                if (isset($functionCallStack[0]) && $functionCallStack[0]->getName() === 'assert') {
+                    $shouldIgnore = true;
+                }
+                break;
+        }
+
+        return $shouldIgnore;
+    }
+}
diff --git a/tests/Unit/PhpStan/IgnoreBooleanAlwaysImpossibleAssertTest.php b/tests/Unit/PhpStan/IgnoreBooleanAlwaysImpossibleAssertTest.php
new file mode 100644
index 00000000..24061629
--- /dev/null
+++ b/tests/Unit/PhpStan/IgnoreBooleanAlwaysImpossibleAssertTest.php
@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\PhpStan;
+
+use PHPStan\Analyser\IgnoreErrorExtension;
+use PHPStan\Testing\RuleTestCase;
+use PHPStan\Rules\Comparison\ImpossibleCheckTypeFunctionCallRule;
+use PHPStan\Rules\Rule;
+
+/**
+ * This covers `function.alreadyNarrowedType` error handling in the `IgnoreBooleanAlways` PHPStan extension class.
+ * `RuleTestCase` allows only one `Rule` class, so separate `TestCase`s are needed for errors generated by other rules.
+ * Note that the base class covers testing the suppression of the warning, and is included via `extends`.
+ * This class covers the non-suppression of warnings in other contexts, which requires detailing the expected warnings.
+ *
+ * @covers \Sabberworm\CSS\PhpStan\IgnoreBooleanAlways
+ */
+final class IgnoreBooleanAlwaysImpossibleAssertTest extends IgnoreBooleanAlwaysTestBase
+{
+    /**
+     * @return ImpossibleCheckTypeFunctionCallRule
+     */
+    protected function getRule(): Rule
+    {
+        // If the class is renamed or removed in the next major release of PHPStan, we'll deal with it then.
+        // @phpstan-ignore phpstanApi.classConstant
+        return self::getContainer()->getByType(ImpossibleCheckTypeFunctionCallRule::class);
+    }
+
+    /**
+     * @test
+     */
+    public function warningIsRetainedForPointlessAssert(): void
+    {
+        // Skip the test in PHP/PHPStan configurations that don't have the required components.
+        // It is good enough to test for those that do.
+        if (!\interface_exists(IgnoreErrorExtension::class)) {
+            self::markTestSkipped('This is testing the testers, and only needs to run whenever possible.');
+        }
+
+        // Second argument is array of expected warnings.
+        $this->analyse(
+            [self::FIXTURES_DIR . 'alwaystrue-pointlessassert.php'],
+            [
+                [
+                    'Call to function is_int() with int will always evaluate to true.',
+                    7,
+                ],
+            ]
+        );
+    }
+}
diff --git a/tests/Unit/PhpStan/IgnoreBooleanAlwaysImpossibleInstanceofTest.php b/tests/Unit/PhpStan/IgnoreBooleanAlwaysImpossibleInstanceofTest.php
new file mode 100644
index 00000000..2f7058f6
--- /dev/null
+++ b/tests/Unit/PhpStan/IgnoreBooleanAlwaysImpossibleInstanceofTest.php
@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\PhpStan;
+
+use PHPStan\Analyser\IgnoreErrorExtension;
+use PHPStan\Testing\RuleTestCase;
+use PHPStan\Rules\Classes\ImpossibleInstanceOfRule;
+use PHPStan\Rules\Rule;
+
+/**
+ * This covers `instanceof.alwaysTrue` error handling in the `IgnoreBooleanAlways` PHPStan extension class.
+ * `RuleTestCase` allows only one `Rule` class, so separate `TestCase`s are needed for errors generated by other rules.
+ * Note that the base class covers testing the suppression of the warning, and is included via `extends`.
+ * This class covers the non-suppression of warnings in other contexts, which requires detailing the expected warnings.
+ *
+ * @covers \Sabberworm\CSS\PhpStan\IgnoreBooleanAlways
+ */
+final class IgnoreBooleanAlwaysImpossibleInstanceofTest extends IgnoreBooleanAlwaysTestBase
+{
+    /**
+     * @return ImpossibleInstanceOfRule
+     */
+    protected function getRule(): Rule
+    {
+        // If the class is renamed or removed in the next major release of PHPStan, we'll deal with it then.
+        // @phpstan-ignore phpstanApi.classConstant
+        return self::getContainer()->getByType(ImpossibleInstanceOfRule::class);
+    }
+
+    /**
+     * @test
+     */
+    public function warningIsRetainedForInstanceofNotInAssert(): void
+    {
+        // Skip the test in PHP/PHPStan configurations that don't have the required components.
+        // It is good enough to test for those that do.
+        if (!\interface_exists(IgnoreErrorExtension::class)) {
+            self::markTestSkipped('This is testing the testers, and only needs to run whenever possible.');
+        }
+
+        // Second argument is array of expected warnings.
+        $this->analyse(
+            [self::FIXTURES_DIR . 'alwaystrue-instanceof-notinassert.php'],
+            [
+                [
+                    'Instanceof between Exception and Exception will always evaluate to true.',
+                    6,
+                ],
+            ]
+        );
+    }
+}
diff --git a/tests/Unit/PhpStan/IgnoreBooleanAlwaysTestBase.php b/tests/Unit/PhpStan/IgnoreBooleanAlwaysTestBase.php
new file mode 100644
index 00000000..59614864
--- /dev/null
+++ b/tests/Unit/PhpStan/IgnoreBooleanAlwaysTestBase.php
@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Unit\PhpStan;
+
+use PHPStan\Analyser\IgnoreErrorExtension;
+use PHPStan\Testing\RuleTestCase;
+use PHPStan\Rules\Rule;
+
+/**
+ * Only one `Rule` can seemingly be covered by classes extending `RuleTestCase`.
+ * This provides some common functionality and settings for `TestCase`s covering `IgnoreBooleanAlways`,
+ * which involves more than one `Rule`.
+ *
+ * @extends RuleTestCase<Rule>
+ */
+abstract class IgnoreBooleanAlwaysTestBase extends RuleTestCase
+{
+    /**
+     * @var non-empty-string
+     */
+    protected const FIXTURES_DIR = __DIR__ . '/../../fixtures/phpstan/';
+
+    /**
+     * @test
+     */
+    public function warningIsIgnoredInAssertInstanceOf(): void
+    {
+        // Skip the test in PHP/PHPStan configurations that don't have the required components.
+        // It is good enough to test for those that do.
+        if (!\interface_exists(IgnoreErrorExtension::class)) {
+            self::markTestSkipped('This is testing the testers, and only needs to run whenever possible.');
+        }
+
+        // Second argument is array of expected warnings.
+        $this->analyse([self::FIXTURES_DIR . 'alwaystrue-instanceof-inassert.php'], []);
+    }
+
+    /**
+     * @return non-empty-array<string>
+     */
+    public static function getAdditionalConfigFiles(): array
+    {
+        return \array_merge(
+            parent::getAdditionalConfigFiles(),
+            [self::FIXTURES_DIR . 'ignorebooleanalways.neon']
+        );
+    }
+}
diff --git a/tests/fixtures/phpstan/alwaystrue-instanceof-inassert.php b/tests/fixtures/phpstan/alwaystrue-instanceof-inassert.php
new file mode 100644
index 00000000..4b979791
--- /dev/null
+++ b/tests/fixtures/phpstan/alwaystrue-instanceof-inassert.php
@@ -0,0 +1,6 @@
+<?php
+
+declare(strict_types=1);
+
+$e = new \Exception();
+\assert($e instanceof \Exception);
diff --git a/tests/fixtures/phpstan/alwaystrue-instanceof-notinassert.php b/tests/fixtures/phpstan/alwaystrue-instanceof-notinassert.php
new file mode 100644
index 00000000..07992813
--- /dev/null
+++ b/tests/fixtures/phpstan/alwaystrue-instanceof-notinassert.php
@@ -0,0 +1,8 @@
+<?php
+
+declare(strict_types=1);
+
+$e = new \Exception();
+if ($e instanceof \Exception) {
+    $theTest = 'passed';
+}
diff --git a/tests/fixtures/phpstan/alwaystrue-pointlessassert.php b/tests/fixtures/phpstan/alwaystrue-pointlessassert.php
new file mode 100644
index 00000000..192c7716
--- /dev/null
+++ b/tests/fixtures/phpstan/alwaystrue-pointlessassert.php
@@ -0,0 +1,8 @@
+<?php
+
+declare(strict_types=1);
+
+function pointless(int $value): void
+{
+    \assert(\is_int($value));
+}
diff --git a/tests/fixtures/phpstan/ignorebooleanalways.neon b/tests/fixtures/phpstan/ignorebooleanalways.neon
new file mode 100644
index 00000000..44918123
--- /dev/null
+++ b/tests/fixtures/phpstan/ignorebooleanalways.neon
@@ -0,0 +1,5 @@
+services:
+  -
+    class: \Sabberworm\CSS\PhpStan\IgnoreBooleanAlways
+    tags:
+      - phpstan.ignoreErrorExtension