diff --git a/docs/docs/050-modules.md b/docs/docs/050-modules.md
index 39f6fe4..d48d8cd 100644
--- a/docs/docs/050-modules.md
+++ b/docs/docs/050-modules.md
@@ -763,3 +763,24 @@ Due to [the limitation of PostHTML](https://github.com/posthtml/htmlnano/issues/
 - `body`
 - `colgroup`
 - `tbody`
+
+### normalizeAttributeValues
+
+Normalize casing of attribute values.
+
+The module won't impact the plain-text size of the output. However it will improve the compression ratio of gzip/brotli used in HTTP compression.
+
+#### Example
+
+Source:
+
+```html
+<form method="GET"></form>
+```
+
+Minified:
+
+```html
+<form method="get"></form>
+```
+
diff --git a/lib/modules/normalizeAttributeValues.es6 b/lib/modules/normalizeAttributeValues.es6
new file mode 100644
index 0000000..2ebf80e
--- /dev/null
+++ b/lib/modules/normalizeAttributeValues.es6
@@ -0,0 +1,61 @@
+const caseInsensitiveAttributes = {
+    autocomplete: ['form'],
+    charset: ['meta', 'script'],
+    contenteditable: null,
+    crossorigin: ['audio', 'img', 'link', 'script', 'video'],
+    dir: null,
+    draggable: null,
+    dropzone: null,
+    formmethod: ['button', 'input'],
+    inputmode: ['input', 'textarea'],
+    kind: ['track'],
+    method: ['form'],
+    preload: ['audio', 'video'],
+    referrerpolicy: ['a', 'area', 'iframe', 'img', 'link'],
+    sandbox: ['iframe'],
+    spellcheck: null,
+    scope: ['th'],
+    shape: ['area'],
+    sizes: ['link'],
+    step: ['input'],
+    translate: null,
+    type: [
+        'a',
+        'link',
+        'button',
+        'embed',
+        'object',
+        'script',
+        'source',
+        'style',
+        'input',
+        'menu',
+        'menuitem'
+    ],
+    wrap: ['textarea']
+};
+
+export default function normalizeAttributeValues(tree) {
+    tree.walk(node => {
+        if (!node.attrs) {
+            return node;
+        }
+
+        Object.entries(node.attrs).forEach(([attrName, attrValue]) => {
+            const attrNameLower = attrName.toLowerCase();
+            if (
+                Object.hasOwnProperty.call(caseInsensitiveAttributes, attrNameLower)
+                && (
+                    caseInsensitiveAttributes[attrNameLower] === null
+                    || caseInsensitiveAttributes[attrNameLower].includes(node.tag)
+                )
+            ) {
+                node.attrs[attrName] = attrValue.toLowerCase();
+            }
+        });
+
+        return node;
+    });
+
+    return tree;
+}
diff --git a/lib/presets/safe.es6 b/lib/presets/safe.es6
index 44983d0..b7e2b6a 100644
--- a/lib/presets/safe.es6
+++ b/lib/presets/safe.es6
@@ -33,6 +33,7 @@ export default {
     },
     minifyConditionalComments: false,
     removeRedundantAttributes: false,
+    normalizeAttributeValues: true,
     removeEmptyAttributes: true,
     removeComments: 'safe',
     removeAttributeQuotes: false,
diff --git a/test/modules/normalizeAttributeValues.js b/test/modules/normalizeAttributeValues.js
new file mode 100644
index 0000000..d99d2e9
--- /dev/null
+++ b/test/modules/normalizeAttributeValues.js
@@ -0,0 +1,14 @@
+import { init } from '../htmlnano';
+import safePreset from '../../lib/presets/safe';
+
+describe('normalizeAttributeValues', () => {
+    const options = safePreset;
+
+    it('default behavior', () => {
+        return init(
+            '<form id="FOo" method="GET"></form>',
+            '<form id="FOo" method="get"></form>',
+            options
+        );
+    });
+});