Sure.
Note: I am not a javascript expert.
There is a way to do this by calling up the File Save As Dialog but I went with the easier download to default browser download location.
Idea is to get godot button, on click, to start downloading file using JavaScript.eval().
First create a temp <a> tag element.
Set it's 'href' attribute to your data (format will depend on what you are serving the user).
Set it's 'download' attribute to a default filename (for example, 'result.png')
Append the temp <a> tag element to your html body
Simulate click using <a> tag .click() method (this will start the download)
Remove temp <a> tag element from html body (clean up)
In my case, serving a png image makes it a bit more tricky since the image has to be converted to a base64 encoding.
For this use the Marshalls Class (Marshalls.raw_to_base64()).
But we cannot directly feed this the Image data since it includes OpenGL texture data and other metadata. So I went ahead and saved the image to the user:// path (this will save it to browser's indexedDB)
Then load this image as File and feed that to .raw_to_base64()
