So like, if you ever need to load a page on a different subdomain to test some kind of origin-y or domainy-y thing, you can just name your test something amazing like
origin-y-test.www.html and it will open the test for you at
www.web-platform.test (rather than
web-platform.test, or similarly, however your system or server is configured).
Now you’ll never need to embed an
<iframe> or call
window.open() ever again (unless you actually need to do those things).