Is it possible to create an HTML canvas without a DOM element?

You can create a new canvas element with document.createElement:

var canvas = document.createElement('canvas');

and then get the context from it. Just make sure you set the width and height. You don't have to add the canvas to the tree in order to make it work:

DEMO

But you definitely have to create that node. You could create a function for that though:

function createContext(width, height) {
    var canvas = document.createElement('canvas');
    canvas.width = width;
    canvas.height = height;
    return canvas.getContext("2d");
}

But that is where my competency ends... whether you can somehow transfer a context to another context or canvas, I don't know...

Its old but what about saving one canvas with toDataURL and copying to the other with drawImage. you could also use save and restore to make a frame buffer

function createCanvas(width, height) {
    var c = document.createElement('canvas');
    c.setAttribute('width', width);
    c.setAttribute('height', height);
    return c;
}

function canvasImg(canvas) {
    var ctx = canvas.getContext('2d');
    ctx.fillRect(0,0,canvas.width, canvas.height);
    var img = canvas.toDataURL('image/png');

    return img;
}

function placeImage(canvas, img) {
    var ctx = canvas.getContext('2d');
    ctx.drawImage(img, 0,0);
}

window.onload = function(){
    var canvas = createCanvas(400, 400);
    var hiddenCanvas = createCanvas(400,400);
    var i = canvasImg(hiddenCanvas);
    var img = new Image();
    img.src = i;
    placeImage(canvas, img);
    document.body.appendChild(canvas);
}

There is apparently a new thing called OffscreenCanvas that was deliberately designed for this use case. An additional bonus is that it also works in Web Workers.

You can read the specifications here: https://html.spec.whatwg.org/multipage/canvas.html#the-offscreencanvas-interface

And see examples here: https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas

Currently it is only fully supported by Chrome and is available behind flags in Firefox and Opera, but you can always check for the latest information on supported browsers here: https://caniuse.com/#feat=offscreencanvas

ps.: Google also has a dedicated guide explaining it's use in Web Workers: https://developers.google.com/web/updates/2018/08/offscreen-canvas