| <!DOCTYPE html> | 
 | <html class="reftest-wait"> | 
 | <meta charset="utf-8"> | 
 | <title>Adoptedstylesheets.concat should work when starting empty</title> | 
 | <link rel="author" href="mailto:masonf@chromium.org"> | 
 | <link rel="help" href="https://drafts.csswg.org/cssom/#extensions-to-the-document-or-shadow-root-interface"> | 
 | <link rel="match" href="CSSStyleSheet-constructable-concat-ref.html"> | 
 |  | 
 | <span>This should be green</span> | 
 | <div id=host></div> | 
 | <pre id=tests>Tests: | 
 | </pre> | 
 |  | 
 | <script> | 
 | function assert_equals(val1,val2) { | 
 |   const t = val1 === val2 ? 'PASS' : `FAIL! ${val1} !== ${val2}`; | 
 |   tests.appendChild(document.createTextNode(`${t}\n`)); | 
 | } | 
 | window.onload = () => { | 
 |   const sheet = new CSSStyleSheet(); | 
 |   sheet.replaceSync('span {background-color:green;}'); | 
 |   assert_equals(document.adoptedStyleSheets.length,0); | 
 |   document.adoptedStyleSheets = document.adoptedStyleSheets.concat([sheet]); | 
 |   assert_equals(document.adoptedStyleSheets.length,1); | 
 |  | 
 |   const host = document.getElementById('host'); | 
 |   const shadow = host.attachShadow({mode: 'open'}); | 
 |   shadow.innerHTML = '<span>This should be green</span>'; | 
 |   assert_equals(shadow.adoptedStyleSheets.length,0); | 
 |   shadow.adoptedStyleSheets = shadow.adoptedStyleSheets.concat([sheet]); | 
 |   assert_equals(shadow.adoptedStyleSheets.length,1); | 
 |   document.documentElement.classList.remove("reftest-wait"); | 
 | }; | 
 | </script> |