gistfile1.txt

<script>
        document.write(screen.width);
        if (screen.width < 980) {
          IWidth = screen.width; 
        } else { IWidth = 980 }
        
document.write('<iframe src="https://mapsengine.google.com/map/embed?mid=zv4x1F3xtt1A.kNa-SYOcLRXY" width="'+IWidth+'" height="680"></iframe>' );
        
</script>

gistfile2.txt

<style>
chapterMap {
	width: 100%; max-width: 980px;
} 
</style>
<iframe id=chapterMap src="https://mapsengine.google.com/map/embed?mid=zv4x1F3xtt1A.kNa-SYOcLRXY"  height="680"></iframe>