would be nice to have a single build mechanism for all versions, ie a single Makefile

About the screen. The r3 code defines SCRW, SCRH. The aspect ratio can be different from the browser window dimensions. How does one stretch (SCRW, SCRH) to fit the browser window correctly?

To stretch the window (but without taking into account the aspect ratio) one can add this to the CSS in the html file.

#canvas {
    position: absolute;
    top: 0px;
    left: 0px;
    margin: 0px;
    width: 100%;
    height: 100%;
    overflow: hidden;
    display: block;
}

It would be nice to have buttons for games so that they are playable on phone/tablet.