diff --git a/test/unit.js b/test/unit.js
index 596da72280..14cf5b6994 100644
--- a/test/unit.js
+++ b/test/unit.js
@@ -27,6 +27,7 @@ import './unit/formats/indent';
 import './unit/formats/list';
 import './unit/formats/bold';
 import './unit/formats/table';
+import './unit/formats/custom-class-blot';
 
 import './unit/modules/clipboard';
 import './unit/modules/history';
diff --git a/test/unit/formats/custom-class-blot.js b/test/unit/formats/custom-class-blot.js
new file mode 100644
index 0000000000..54d676a921
--- /dev/null
+++ b/test/unit/formats/custom-class-blot.js
@@ -0,0 +1,86 @@
+import Quill from '../../../core/quill';
+
+function createClassDataBlot(BaseBlot, tagName) {
+  return class ClassDataBlot extends BaseBlot {
+    static create(value) {
+      const node = super.create(value);
+      if (value && value.customClass) node.className = value.customClass;
+      if (value && value.customData) node.dataset.test = value.customData;
+      return node;
+    }
+
+    static formats(node) {
+      if (node.tagName !== tagName) return false;
+
+      const formats = {};
+      if (node.className) formats.customClass = node.className;
+      if (node.dataset.test) formats.customData = node.dataset.test;
+
+      return formats;
+    }
+  };
+}
+
+describe('Custom class/data blots', function () {
+  const InlineBlot = Quill.import('blots/inline');
+  const BlockBlot = Quill.import('blots/block');
+
+  const ClassInline = createClassDataBlot(InlineBlot, 'SPAN');
+  ClassInline.tagName = 'SPAN';
+
+  const ClassBlock = createClassDataBlot(BlockBlot, 'P');
+  ClassBlock.tagName = 'P';
+
+  it('Inline: create() applies class and data; formats() extracts them back', function () {
+    const node = ClassInline.create({ customClass: 'testClass', customData: 'testData' });
+
+    expect(node.outerHTML).toEqualHTML('<span class="testClass" data-test="testData"></span>');
+    expect(ClassInline.formats(node)).toEqual({ customClass: 'testClass', customData: 'testData' });
+  });
+
+  it('Block: create() applies class and data; formats() extracts them back', function () {
+    const node = ClassBlock.create({ customClass: 'testClass', customData: 'testData' });
+
+    expect(node.outerHTML).toEqualHTML(
+      '<p class="testClass" data-test="testData"></p>',
+    );
+
+    expect(ClassBlock.formats(node)).toEqual({
+      customClass: 'testClass',
+      customData: 'testData',
+    });
+  });
+
+  it('formats() returns false for unexpected tags', function () {
+    const div = document.createElement('div');
+    div.className = 'testClass';
+    div.dataset.test = 'testData';
+
+    expect(ClassInline.formats(div)).toBe(false);
+
+    const span = document.createElement('span');
+    span.className = 'testClass';
+    span.dataset.test = 'testData';
+
+    expect(ClassBlock.formats(span)).toBe(false);
+  });
+
+  it('Custom inline tag (<i>): create() and formats() work correctly', function () {
+    const ItalicBlot = createClassDataBlot(InlineBlot, 'I');
+    ItalicBlot.tagName = 'I';
+
+    const node = ItalicBlot.create({
+      customClass: 'testClass',
+      customData: 'testData',
+    });
+
+    expect(node.outerHTML).toEqualHTML(
+      '<i class="testClass" data-test="testData"></i>',
+    );
+
+    expect(ItalicBlot.formats(node)).toEqual({
+      customClass: 'testClass',
+      customData: 'testData',
+    });
+  });
+});