This jQuery solution works for me in Chrome and Firefox, for both Ctrl+S and Cmd+S.
$(document).keydown(function(e) {
var key = undefined;
var possible = [ e.key, e.keyIdentifier, e.keyCode, e.which ];
while (key === undefined && possible.length > 0)
{
key = possible.pop();
}
if (key && (key == '115' || key == '83' ) && (e.ctrlKey || e.metaKey) && !(e.altKey))
{
e.preventDefault();
alert("Ctrl-s pressed");
return false;
}
return true;
});