Here are a collection of simple WebMetal examples. They should work in the latest Safari Technology Preview releases, as well as nightly WebKit builds. At the moment there are a few small differences between the implementation and the documented proposal, most of which are documented in the code.
Make sure you are on a system with WebMetal enabled. In Safari, first make sure the Developer Menu is visible (Preferences → Advanced), then Develop → Experimental Features → Enable WebMetal.
WebMetal is an experimental technology, and you should not browse the entire Web with it enabled for now. It doesn't do any validation of content, and thus may cause some issues with your computer.