IE 11 doesn't support arrow functions. Transpile your code to ES5 compliant.
You can use an online JavaScript transpiler like BableJS (https://babeljs.io/), resulting into:
"use strict";
html2canvas(document.body).then(function (canvas) {
document.body.appendChild(canvas);
});
Most helpful comment
You can use an online JavaScript transpiler like BableJS (https://babeljs.io/), resulting into:
"use strict";
html2canvas(document.body).then(function (canvas) {
document.body.appendChild(canvas);
});