In #7 I proposed sharing Blobs. The primary use case for this is sharing images, but using this API to share images is a bit hard. If you wanted to share the contents of a <canvas> you would do:

var canvas;  // HTMLCanvasElement representing a <canvas> element
canvas.toBlob(blob => navigator.share({blob: blob, mimeType: 'image/png'}),
              'image/png');

A little unwieldy, but manageable.

To share an <img> element, things get a bit hairy:

var image;  // HTMLImageElement representing an <img> element
var canvas = document.createElement('canvas');
canvas.width = image.width;
canvas.height = image.height;
canvas.getContext('2d').drawImage(image, 0, 0);
canvas.toBlob(blob => navigator.share({blob: blob, mimeType: 'image/png'}),
              'image/png');

(Note: I haven't tested this code, just wrote it on the fly. There may be an easier way but that's the best way I can find to convert an image into a blob.)

So maybe we should be providing a share attribute called 'image' that takes a CanvasImageSource -- basically lets you directly share an Image, Video or Canvas object without having to convert to a blob. The usage would be:

var image;  // HTMLImageElement representing an <img> element
navigator.share({image: image, mimeType: 'image/png'}),

As shown above, this isn't strictly necessary (can be polyfilled). So it would be a policy decision whether we want to provide unnecessary-but-helpful abstractions, or whether we should save that for wrapper libraries.