[1] In older browsers that don\'t support the async attribute, parser-inserted scripts block the parser; script-inserted scripts execute asynchronously in IE and WebKit, but synchronously in Opera and pre-4.0 Firefox. In Firefox 4.0, the async DOM property defaults to true for script-created scripts, so the default behavior matches the behavior of IE and WebKit. To request script-inserted external scripts be executed in the insertion order in browsers where the document.createElement(\"script\").async evaluates to true (such as Firefox 4.0), set .async=false on the scripts you want to maintain order. Never call document.write() from an async script. In Gecko 1.9.2, calling document.write() has an unpredictable effect. In Gecko 2.0, calling document.write() from an async script has no effect (other than printing a warning to the error console).

[2] Starting in Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1), inserting script elements that have been created by calling document.createElement(\"script\") into the DOM no longer enforces execution in insertion order. This change lets Gecko properly abide by the HTML5 specification. To make script-inserted external scripts execute in their insertion order, set .async=false on them.

Also, script elements inside iframe , noembed and noframes elements are now executed, for the same reasons.

[3] In versions prior to Internet Explorer 10 Trident implemented script by a proprietary specification. Since version 10 it conforms to the W3C specification.

[4] The crossorigin attribute was implemented in WebKit in WebKit bug 81438.

[5] WebKit bug 148363 tracks WebKit implementation of Subresource Integrity (which includes the integrity attribute).

[6]从Gecko 1.9.2 (Firefox 3.6 / Thunderbird 3.1 / Fennec 1.0)开始，在没有src属性的脚本标签上的defer属性会被忽略。但是在Gecko 1.9.1 (Firefox 3.5 / Thunderbird 3.0 / SeaMonkey 2.0)中，如果定义了 defer属性，即使内嵌的脚本也会被延迟执行。