Use documentFragment instead of directly appending element

instant-flat-ui.js

var f = document.createDocumentFragment();
var d = document.createElement('div');
d.innerHTML = '\
    <style>\
        *:not(.icon):not(i), *:not(.icon):not(i):after, *:not(.icon):not(i):before {\
            box-shadow: none !important;\
            text-shadow: none !important;\
            background-image: none !important;\
        }\
        *:not(.icon):not(i) {\
            border-color: transparent !important;\
        }\
    </style>\
';
f.appendChild(d);
document.body.appendChild(f);