Thinking about a rendering framework in js, following http://blog.jochentopf.com/2011-03-22-new-approaches-for-map-rendering.html

renderer.js

var style = require('./style.js');

// postgres reader
var r = new PostgresReader({
    db: 'mapnik', 
    user: 'osm', 
    pass: 'pass', 
    
    // one could query imposm-like tables, too
    tables: {
        point: {
            name: 'planet_osm_point', 
            select: '*', 
            geom: 'way'
        }, 
        line: {
            name: 'planet_osm_line', 
            select: '*', 
            geom: 'way'
        }
    }
});

// canvas to paint to
var c = new Canvas();

style.apply(r, c);

r.on('ended', function() {
    c.save('out.png');
});
r.start();

style.js

exports.apply = function(r, c) {
	// some boundingbox
	var germany = new BBox(1, 2, 3, 4);

	// render cities
	r.on('point').tag({'place': 'city'}).on(function(point) {
		
		c.drawText({
			text: point.attr.title, 
			geom: point.geom, 
			style: {
				// use a different color in germany
				'font-color': germany.contains(point.geom) ? 'blue' : 'red'
			}
		});
	});

	// alternatively one could specify a different rule for germany using the filter chain

	// render villages inside germany
	r.on('point').tag({'place': 'village'}).inside(germany).on(function(point) {
		
		c.drawText({
			text: point.title, 
			geom: point.geom, 
			style: {
				'font-color': 'green'
			}
		});
	});

	// render villages outside germany
	r.on('point').tag({'place': 'village'}).outside(germany).on(function(point) {
		
		c.drawText({
			text: point.attr.title, 
			geom: point.geom, 
			style: {
				'font-color': 'yellow'
			}
		});
	});

	// add highways to highway grid
	var highways = new Grid();

	r.on('line').tag('highway').on(function(line) {
		highways.add(line);
	});

	// render highway grid
	r.on('ending', function() {
		c.drawGrid({
			grid: highways, 
			style: {
				'stroke-color': 'red'
			}
		});
	});
}