CSS3 transitions have some significant limitations that make them unworkable for a serious animation platform. They don’t provide precise controls over the timing or easing. They’re great for simple effects but the GreenSock Animation Platform delivers extremely precise rendering, so you can do things like pause() and reverse() an animation anytime or skip to a specific time and play from there, etc. Try creating a CSS3 transition that uses an elastic.out or slow motion ease and then jump to 0.72494-seconds into a 2-second transition and pause() only to resume() later. It’s impossible from what I understand. So no, the platform doesn’t make use of CSS3 transitions. However, it is highly optimized for performance. See the detailed cage match where GSAP battles CSS3 transitions where there’s a detailed comparison in several categories.

GSAP itself is pure JavaScript and should work in virtually ALL browsers. GSAP wasn’t intended to solve all browser incompatibilities, but it does implement wizardry for critical features like opacity, transforms (rotation, scaleX, scaleY, skewX, skewY, x, and y), and transformOrigin so those should work in all major browsers even back to IE6. Firefox doesn’t support backgroundPositionX or backgroundPositionY, so those specific properties won’t work but backgroundPosition will for virtually all browsers. There is NOT a predetermined list of css properties that you can tween – the platform will attempt to tween ANY property you pass in. If it is numeric, it will tween it. If it isn’t numeric and it isn’t a recognized special property, CSSPlugin will just set the property to the value you provide (without tweening it). So, for example, if you try to tween to display:"inline", that isn’t a tweenable property but it will still be set accordingly during the tween, so feel free to use that to your advantage.

IntertiaPlugin is a membership benefit of Club GreenSock, so you need to sign up to get that. Draggable, however, is included in all the standard downloads and you don't need a membership to get that; Draggable can be used apart from InertiaPlugin as long as you don't need the momentum-based motion. GreenSock's standard license allows you to use GreenSock tools for free in everything except a very specific type of commercial project (if you collect a fee from multiple customers for the app/product/game/site that uses GreenSock tools) which makes it extremely accessible and business-friendly while providing a small funding mechanism to sustain ongoing support, enhancement, and innovation. The web is littered with abandoned “open source” projects, but GreenSock has a years-long track record of commitment to the platform. This unique licensing model is a key component of its sustainability. If you charge multiple customers a usage/access/license fee, please simply sign up for a “Business Green” Club GreenSock membership which comes with a special commercial license granting you permission to do so. Click here for details. Joining the club also gets you members-only bonus plugins, utilities, update notifications, and more.