As an internal implementation detail for sharing code across browsers, that sounds compelling enough. Web developers would never need to no about it though, as

As an internal implementation detail for sharing code across browsers, that sounds compelling enough. Web developers would never need to no about it though, as its just part of a browser's rendering flow that happens to be shared by browser vendors. In reality though, I don't see browser vendors playing well to that level. Sharing specs is slow, sharing rendering code would be a nightmare. Beyond that, I've usually seen Houdini demoed with developers focused features, effectively adding small bits of your own logic into CSS rendering. That pulls it out of the shared library and into userland.