The new Gaia multi-page architecture would benefit from easily being able to transition when navigating to a new page. Currently, transitions like this in a single-frame application are very limited, as loading one page will remove the previous page and there is no way to animate between these two states.
This page will track the ongoing effort to provide an API to enable transitioning from one page to another.
There are currently two proposals, one from Google and another suggested by a Mozilla employee. The former Google proposition is based around Android's similar transition capabilities and API, and integrates well with Android. The latter is easier to implement in the platform and possibly easier to author simple transitions in for web developers. The latter also contains a working proof-of-concept that can be used to perform navigation transitions in a slightly more limited capacity than the spec outlines, but without platform support.
Discussion is still ongoing and both proposals are likely to change.