Here's a hello world for X3D: https://www.web3d.org/x3d/content/examples/Basic/X3dSpecific...
And here's a hello world for OpenUSD: https://www.openusd.org/release/tut_helloworld.html#viewing-...
Although X3D is a bit more verbose, a lot of parallels can be drawn between the two, 20 years apart.