CSSOM From Houdini Spec

[FlorianRappl]

Houdini is getting more in shape giving us some of the spec details that are particularly interesting, e.g.,

• https://www.w3.org/TR/css-typed-om-1/ (CSS Typed OM)
• https://www.w3.org/TR/css-properties-values-api-1/ (CSS Properties Values API)

These two specs define not only how values are represented in IDL, but also how to extend / define properties. Those things are already somehow in AngleSharp.Css but should be made conforming to those specs to be on the same track early.