test(v4): cover preflight and layer ordering (#83)

## Summary

- Add Playwright test verifying preflight base styles (body, button,
list) are applied via the virtual global CSS injected by the plugin.
- Add Playwright test verifying explicit @layer ordering () controls
which declarations win across layers.

Author: colinaaa

test/basic/index.test.ts

@@ -25,6 +25,97 @@ test('should build with tailwind utilities', async ({ page }) => {
   await server.close();
 });
 
+test('preflight base styles are applied from virtual global CSS', async ({
+  page,
+}) => {
+  const rsbuild = await createRsbuild({
+    cwd: __dirname,
+    rsbuildConfig: {
+      plugins: [pluginTailwindCSS()],
+    },
+  });
+
+  await rsbuild.build();
+  const { server, urls } = await rsbuild.preview();
+
+  try {
+    await page.goto(urls[0]);
+
+    await page.evaluate(() => {
+      const button = document.createElement('button');
+      button.id = 'preflight-button';
+      button.textContent = 'Button';
+      document.body.appendChild(button);
+
+      const list = document.createElement('ul');
+      list.id = 'preflight-list';
+      const item = document.createElement('li');
+      item.textContent = 'Item';
+      list.appendChild(item);
+      document.body.appendChild(list);
+    });
+
+    await expect(page.locator('body')).toHaveCSS('margin', '0px');
+    await expect(page.locator('#preflight-button')).toHaveCSS(
+      'border-radius',
+      '0px',
+    );
+    await expect(page.locator('#preflight-button')).toHaveCSS(
+      'background-color',
+      'rgba(0, 0, 0, 0)',
+    );
+    await expect(page.locator('#preflight-list')).toHaveCSS(
+      'list-style-type',
+      'none',
+    );
+  } finally {
+    await server.close();
+  }
+});
+
+test('layer order controls which styles win across layers', async ({
+  page,
+}) => {
+  const rsbuild = await createRsbuild({
+    cwd: __dirname,
+    rsbuildConfig: {
+      plugins: [pluginTailwindCSS()],
+    },
+  });
+
+  await rsbuild.build();
+  const { server, urls } = await rsbuild.preview();
+
+  try {
+    await page.goto(urls[0]);
+
+    await page.addStyleTag({
+      content: `@layer utilities {
+  #layer-order-test { color: red; }
+}
+
+@layer base {
+  #layer-order-test { color: blue; }
+}
+`,
+    });
+
+    await page.evaluate(() => {
+      const el = document.createElement('div');
+      el.id = 'layer-order-test';
+      el.textContent = 'Layer order';
+      document.body.appendChild(el);
+    });
+
+    await expect(page.locator('#layer-order-test')).toHaveCSS(
+      'color',
+      'rgb(255, 0, 0)',
+    );
+  } finally {
+    await server.close();
+  }
+});
+
 test('should not generate tailwind.config.js in dist/', async () => {
   const rsbuild = await createRsbuild({
     cwd: __dirname,