diff --git a/CHANGELOG.md b/CHANGELOG.md
index c01983fd..1844e90c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,7 +6,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ## x.y.z
 
 ### Added
-
+- Add support for inserting an item in a CSS list (#545)
 - Add a class diagram to the README (#482)
 - Add support for the `dvh`, `lvh` and `svh` length units (#415)
 - Add more tests (#449)
diff --git a/src/CSSList/CSSList.php b/src/CSSList/CSSList.php
index 9ce207cf..b107b6ab 100644
--- a/src/CSSList/CSSList.php
+++ b/src/CSSList/CSSList.php
@@ -294,6 +294,22 @@ public function splice($iOffset, $iLength = null, $mReplacement = null)
         array_splice($this->aContents, $iOffset, $iLength, $mReplacement);
     }
 
+    /**
+     * Inserts an item in the CSS list before its sibling. If the desired sibling cannot be found,
+     * the item is appended at the end.
+     *
+     * @param RuleSet|CSSList|Import|Charset $item
+     * @param RuleSet|CSSList|Import|Charset $sibling
+     */
+    public function insertBefore($item, $sibling): void
+    {
+        if (in_array($sibling, $this->aContents, true)) {
+            $this->replace($sibling, [$item, $sibling]);
+        } else {
+            $this->append($item);
+        }
+    }
+
     /**
      * Removes an item from the CSS list.
      *
diff --git a/tests/CSSList/DocumentTest.php b/tests/CSSList/DocumentTest.php
index 10f72e3e..ec84f1e2 100644
--- a/tests/CSSList/DocumentTest.php
+++ b/tests/CSSList/DocumentTest.php
@@ -85,4 +85,59 @@ public function setContentsReplacesContentsSetInPreviousCall(): void
 
         self::assertSame($contents2, $this->subject->getContents());
     }
+
+    /**
+     * @test
+     */
+    public function insertContentBeforeInsertsContentBeforeSibbling()
+    {
+        $bogusOne = new DeclarationBlock();
+        $bogusOne->setSelectors('.bogus-one');
+        $bogusTwo = new DeclarationBlock();
+        $bogusTwo->setSelectors('.bogus-two');
+
+        $item = new DeclarationBlock();
+        $item->setSelectors('.item');
+
+        $sibling = new DeclarationBlock();
+        $sibling->setSelectors('.sibling');
+
+        $this->subject->setContents([$bogusOne, $sibling, $bogusTwo]);
+
+        self::assertCount(3, $this->subject->getContents());
+
+        $this->subject->insertBefore($item, $sibling);
+
+        self::assertCount(4, $this->subject->getContents());
+        self::assertSame([$bogusOne, $item, $sibling, $bogusTwo], $this->subject->getContents());
+    }
+
+    /**
+     * @test
+     */
+    public function insertContentBeforeAppendsIfSibblingNotFound()
+    {
+        $bogusOne = new DeclarationBlock();
+        $bogusOne->setSelectors('.bogus-one');
+        $bogusTwo = new DeclarationBlock();
+        $bogusTwo->setSelectors('.bogus-two');
+
+        $item = new DeclarationBlock();
+        $item->setSelectors('.item');
+
+        $sibling = new DeclarationBlock();
+        $sibling->setSelectors('.sibling');
+
+        $orphan = new DeclarationBlock();
+        $orphan->setSelectors('.forever-alone');
+
+        $this->subject->setContents([$bogusOne, $sibling, $bogusTwo]);
+
+        self::assertCount(3, $this->subject->getContents());
+
+        $this->subject->insertBefore($item, $orphan);
+
+        self::assertCount(4, $this->subject->getContents());
+        self::assertSame([$bogusOne, $sibling, $bogusTwo, $item], $this->subject->getContents());
+    }
 }