http://ie.microsoft.com/testdrive/Performance/AsyncScripts/Default.html

The HTML5 "async" attribute simplifies page-load performance improvements and dynamic script loading. Load Independent Scripts Faster ▶ Run Test Traditional scripts block the HTML parser, preventing the rest of the page from being rendered until the script has been downloaded and run. Scripts using HTML5 async unblock the rest of the page so it can continue loading while the script is being downlo

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script async属性を視覚的に確認できるテストページ