Start reworking description of property types and animation types

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branch : rework-animation-model
Rename animation type to animation behavior

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branch : rework-animation-model
Add string and real number animation behaviors

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branch : rework-animation-model
Add length/percentage/calc and color animation behaviors

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branch : rework-animation-model
Tidy up color math

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branch : rework-animation-model
Add animatable as transform list and 'other'

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branch : rework-animation-model
Revise section on accumulation

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branch : rework-animation-model
Remove old section on calculating lengths

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branch : rework-animation-model
Rework section on combining animations to describe animating DOM attributes

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branch : rework-animation-model
Fix a broken link

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branch : rework-animation-model
Remove redundant animatable properties section

This content is now covered in the initial description

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branch : rework-animation-model
Add note about the range of p in interpolation procedures

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branch : rework-animation-model
Remove timing function chains

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branch : rework-animation-model
Add per-keyframe timing functions

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branch : rework-animation-model
Rework keyframe normalization

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branch : rework-animation-model
Add unified spacing algorithm for keyframes

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branch : rework-animation-model
Add spacing member to KeyframeEffect and make constructor use a KeyframeEffectOptions parameter

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branch : rework-animation-model
Remove old section on paced timing functions

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branch : rework-animation-model
Fix a broken reference

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branch : rework-animation-model
Add non-normative description of spacing modes

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branch : rework-animation-model