14. A device, comprising: at least one processor and a memory; the at least one processor configured to: select an animation from a web page; extract from a document closure for the web page, pieces of documents of the document closure, the extracted pieces pertaining to the selected animation, the document closure including at least one content document and at least one presentation document; create a composite data structure from the extracted pieces, the composite data structure differs from the web page; create a complete representation of the selected animation from the composite data structure; play a preview of the animation from the complete representation of the selected animation; in response to receiving input manipulating the preview, convert the input to changes to properties of the animation; update corresponding properties in the composite 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 composite data structure with an indication of the winning properties.
14. A device, comprising: at least one processor and a memory; the at least one processor configured to: select an animation from a web page; extract from a document closure for the web page, pieces of documents of the document closure, the extracted pieces pertaining to the selected animation, the document closure including at least one content document and at least one presentation document; create a composite data structure from the extracted pieces, the composite data structure differs from the web page; create a complete representation of the selected animation from the composite data structure; play a preview of the animation from the complete representation of the selected animation; in response to receiving input manipulating the preview, convert the input to changes to properties of the animation; update corresponding properties in the composite 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 composite data structure with an indication of the winning properties. 17. The device of claim 14 , wherein the at least one processor is further configured to: select properties and @keyframe rules affecting the selected animation from the document closure for the web page; and combine the selected properties and the selected @keyframe rules into the composite data structure.