diff --git a/packages/hint-highest-available-document-mode/README.md b/packages/hint-highest-available-document-mode/README.md
index aa372d3c1b2..9e2bf48f8f5 100644
--- a/packages/hint-highest-available-document-mode/README.md
+++ b/packages/hint-highest-available-document-mode/README.md
@@ -46,10 +46,23 @@ Notes:
 
 ## What does the hint check?
 
-By default, the hint checks if the `X-UA-Compatible` response header
-is sent with the value of `IE=edge`, and that the `meta` tag isn’t
-used.
+### Important note about case-sensitivity
 
+The `X-UA-Compatible` header is defined as case-insensitive according to Microsoft's specification.
+
+The hint now correctly performs a **case-insensitive comparison** of the value.
+This means the following are all treated as valid:
+
+    X-UA-Compatible: IE=edge
+    X-UA-Compatible: Ie=EdGe
+    x-ua-compatible: ie=edge
+
+Only values that are not equal to `ie=edge` (ignoring case and spacing)
+will trigger an error.
+
+```text
+X-UA-Compatible: IE=edge
+```
 ### Examples that **trigger** the hint for defaults
 
 `X-UA-Compatible` response header is not sent:
diff --git a/packages/hint-highest-available-document-mode/src/hint.ts b/packages/hint-highest-available-document-mode/src/hint.ts
index b697c20085d..df7d349e2ce 100644
--- a/packages/hint-highest-available-document-mode/src/hint.ts
+++ b/packages/hint-highest-available-document-mode/src/hint.ts
@@ -80,7 +80,7 @@ export default class HighestAvailableDocumentModeHint implements IHint {
                 return;
             }
 
-            if (headerValue !== 'ie=edge') {
+            if (headerValue?.trim().toLowerCase() !== 'ie=edge') {
                 context.report(
                     resource,
                     getMessage('headerValueShouldBe', context.language),