|  | <!DOCTYPE html> | 
|  | <html class="reftest-wait foo"> | 
|  | <title>View transitions: ensure :only-child is supported on view-transition</title> | 
|  | <link rel="help" href="https://drafts.csswg.org/css-view-transitions-1/"> | 
|  | <link rel="author" href="mailto:khushalsagar@chromium.org"> | 
|  |  | 
|  | <script src="/resources/testharness.js"></script> | 
|  | <script src="/resources/testharnessreport.js"></script> | 
|  |  | 
|  | <style> | 
|  | ::view-transition { | 
|  | background-color: red; | 
|  | } | 
|  |  | 
|  | html:only-child { | 
|  | background-color: blue; | 
|  | } | 
|  |  | 
|  | :root:only-child { | 
|  | background-color: blue; | 
|  | } | 
|  |  | 
|  | :only-child { | 
|  | background-color: blue; | 
|  | } | 
|  |  | 
|  | .foo:only-child { | 
|  | background-color: blue; | 
|  | } | 
|  | </style> | 
|  |  | 
|  | <script> | 
|  | promise_test(() => { | 
|  | assert_implements(document.startViewTransition, "Missing document.startViewTransition"); | 
|  | return new Promise(async (resolve, reject) => { | 
|  | let transition = document.startViewTransition(); | 
|  | transition.ready.then(() => { | 
|  | let style = getComputedStyle( | 
|  | document.documentElement, "::view-transition"); | 
|  | if (style.backgroundColor == "rgb(255, 0, 0)") | 
|  | resolve(); | 
|  | else | 
|  | reject(style.backgroundColor); | 
|  | }); | 
|  | }); | 
|  | }, ":only-child is not supported on view-transition"); | 
|  | </script> |