1_load_html2canvas.js

let s = document.createElement("script");
s.src = "https://html2canvas.hertzen.com/dist/html2canvas.js";
document.body.append(s);

2_load_file-saver.js

let s2 = document.createElement("script");
s2.src = "https://cdn.jsdelivr.net/npm/file-saver@2.0.2/dist/FileSaver.min.js";
document.body.append(s2);

3_save.js

html2canvas(document.body, {allowTaint: true, useCORS: true})
.then(function(canvas) {
  canvas.toBlob(function(blob){ saveAs(blob,"image.png"); });
})
.catch(function(e){ console.error(e)});