1. A system comprising: at least one processor of a computing device; a memory of the computing device; and an animation development tool comprising at least one program module that causes the at least one processor to: receive a document closure for a web page, the web page including an animation, the document closure including at least one content file and at least one presentation file; extract from the document closure, pieces of documents of the document closure, into a data structure representing the animation, the extracted pieces pertaining to the animation, the data structure differs from the web page; preview the animation in the animation development tool from the data structure; and convert user gestures manipulating properties of the animation to changes to the data structure; generate an internal set of CSS rules to apply to the animation that because of CSS precedence rules, would not play when an application including the animation is run; compute a set of winning animation properties for the animation; and update the data structure with an indication of the winning properties.