They have been around for several years, yet I’ve never had an occasion to actually use them, and thus had no idea what that syntax meant. Apparently they are pretty mainstream in certain communities (e.g. ones that use graphql or styled components).
Example from Relay:
Relay.QL`
fragment on Greetings {
hello,
}
`;
Example from styled components:
styled.h1` font-size: 1.5em; text-align: center; color: palevioletred; `;
Article on MDN.io
