Hi, GreenSock.
I user of GreenSock for year.
GSAP is more size (~400Kb source), highly compatibility.
High-performance, re-markly compatible, powerful modules/plugins.
But for the Web needs more methods in last year, like CSS3 Filter, CSS3 PIE Plugin, Relative values (+, -, *, /, cos, sin, tan, d2r, etc).
I maked own JS Tweening engine, Tween.js Modded, source ~30Kb (http://tweenjs.ga) and supports CSS3 PIE, CSS3 Filters, relative values only +,-,*,/. Also it's supports string tweening and makes highly compatible and performance like TweenMax.