The feature of noVNC is "Clipboard copy/paste", why not use "clipboardData" to get the content of the clipboard of the browser, but use the textarea of html to get it.
function handlePaste (e) {
var clipboardData, pastedData;
// Stop data actually being pasted into div
e.stopPropagation();
e.preventDefault();
// Get pasted data via clipboard API
clipboardData = e.clipboardData || window.clipboardData;
pastedData = clipboardData.getData('Text');
// Do whatever with pasteddata
console.log('patse', pastedData);
}
document.addEventListener('paste', handlePaste);