question: Possible to implement anything similar to Raphael's customAttributes? about svg.js

[wout]

You can achieve similar functionality with svg.js, although it takes a completely different approach. Let's take something similar to Raphael's hsb example for comparison.

/* extend shape module with hsb method */
SVG.extend(SVG.Shape, {
  hsb: function(h, s, b) {
    return this.attr('fill', { h: h % 360, s: s, b: b })
  }
})

/* create canvas */
var draw = svg('canvas')

/* create circle */
var c = draw.circle(100).move(10, 10).hsb(0, 100, 100)

/* animate hsb */
c.animate(1e4, '-').during(function(pos, morph) {
  this.hsb(morph(0,360), 100, 100)
})

It wouldn't be too hard to write a plugin to bend this functionality to a more Raphael-like syntax.
But that's a different story.

More info on extending objects in svg.js can be found here:
https://github.com/wout/svg.js#extending-functionality

And info on synchronising animations:
https://github.com/wout/svg.js#synchronising-animations

To come to your second question, I have indeed looked into requestAnimationFrame and decided to go with a interval / date iterator to keep it simple. But I am looking to replace it with rAF as a part of performance enhancements as soon as the library gets to a steady level structurally.