script[defer] doesn't work in IE<=9 · Issue #42 · h5bp/lazyweb-requests

And the first script modifies the dom with appendChild, innerHTML, (etc.), the second script can start executing before the first one has finished. Thus, a dependency between the two will break. The details of this limitation begin at this comment This essentially means that script[defer] cannot be used in most cases unless you have dropped IE8 and IE9 support. If, however, you can UA sniff to ser

[teppeis]

IE9以下でdefer付きで呼ばれるscriptの実行順序が保証されないケースがある。先に呼ばれたappendChild, innerHTMLをいじると発生する可能性がある。