XRTL is still in its very early stages and appears to be primarily the work of Google's Ben Vanik. From the project's GitHub repository, "A lightweight framework for writing efficient cross-platform rendering code in modern C++. It abstracts operating system and toolchain concepts to enable quick prototyping and interation of rich interactive applications that run beautifully everywhere. The library is engineered for small code sizes to enable fast loading and low resource usage even within web browsers."
Vulkan and OpenGL are among the graphics targets and there are plans for Android and iOS support. EmScripten support also appears planned for web browser support.
The work is still in its very early stages, so there isn't much documentation right now, but it should be an interesting project to watch in the future. Currently it can be built for macOS, Linux, and Windows.