docs: add part="text" styling section to README and test for aria-hidden coexistence

Document the shadow DOM `part="text"` attribute in a new Styling section
with a CSS `::part()` usage example. Add test coverage verifying
`part="text"` is preserved when `aria-hidden="true"` is also set.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: silverwind

README.md

@@ -284,6 +284,17 @@ Lang is a [built-in global attribute](https://developer.mozilla.org/en-US/docs/W
 
 Adding the `no-title` attribute will remove the `title` attribute from the `<relative-time>` element. The `title` attribute is inaccessible to screen reader and keyboard users, so not adding a title attribute allows a user to create a custom, accessible tooltip if one is desired.
 
+## Styling
+
+The element renders its text inside a Shadow DOM `<span>` with a [`part="text"`](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/part) attribute. This allows you to style the displayed text from outside the shadow root using the `::part()` CSS pseudo-element:
+
+```css
+relative-time::part(text) {
+  color: rebeccapurple;
+  font-weight: bold;
+}
+```
+
 ## Browser Support
 
 Browsers without native [custom element support][support] require a [polyfill][ce-polyfill].

test/relative-time.js

@@ -2143,6 +2143,18 @@ suite('relative-time', function () {
       const span = time.shadowRoot.querySelector('span')
       assert.equal(span.getAttribute('part'), 'text')
     })
+
+    test('shadow root span has part="text" alongside aria-hidden="true"', async () => {
+      const now = new Date().toISOString()
+      const time = document.createElement('relative-time')
+      time.setAttribute('datetime', now)
+      time.setAttribute('aria-hidden', 'true')
+      await Promise.resolve()
+
+      const span = time.shadowRoot.querySelector('span')
+      assert.equal(span.getAttribute('part'), 'text')
+      assert.equal(span.getAttribute('aria-hidden'), 'true')
+    })
   })
 
   suite('legacy formats', function () {