15. One or more non-transitory computer-readable media storing one or more sequences of instructions which, when executed by one or more processors, cause a computing device to perform a method comprising: displaying a first timeline for a first animation object; displaying a second timeline for a second animation object; displaying a first identifier of the first animation object in association with the first timeline; displaying a second identifier of the second animation object in association with the second timeline; wherein the first identifier and the second identifier are displayed with respect to each other in an order that determines a z-index order of the first animation object with respect to the second animation object in a style sheet animation of at least the first animation object and the second animation object; generating style sheet language text which, when processed by a style sheet animation processor, causes the first animation object and the second animation object to be animated in the style sheet animation in accordance with the determined z-index order.
15. One or more non-transitory computer-readable media storing one or more sequences of instructions which, when executed by one or more processors, cause a computing device to perform a method comprising: displaying a first timeline for a first animation object; displaying a second timeline for a second animation object; displaying a first identifier of the first animation object in association with the first timeline; displaying a second identifier of the second animation object in association with the second timeline; wherein the first identifier and the second identifier are displayed with respect to each other in an order that determines a z-index order of the first animation object with respect to the second animation object in a style sheet animation of at least the first animation object and the second animation object; generating style sheet language text which, when processed by a style sheet animation processor, causes the first animation object and the second animation object to be animated in the style sheet animation in accordance with the determined z-index order. 16. The one or more non-transitory computer-readable media of claim 15 , the method further comprising: in response to obtaining first input indicating a first point along the first timeline, displaying a first keyframe indicator on the first timeline at the first point; wherein the first keyframe indicator represents a first keyframe of the style sheet animation; wherein the first point corresponds to a first time during the style sheet animation; in response to obtaining second input indicating a second point along the first timeline, displaying a second keyframe indicator on the first timeline at the second point; wherein the second keyframe indicator represents a second keyframe of the style sheet animation; wherein the second point corresponds to a second time during the style sheet animation; wherein generating the style sheet language text further comprises generating style sheet language text that declares the first and second keyframes and that reflects an association between the first keyframe and the first time and an association between the second keyframe and the second time.