Use Case:

Some web applications will serve multiple domains and render different
content depending on the incoming header. In a development environment,
these applications may use localhost subdomains to simulate these
alternative domains.

To reduce overall configuration, make the webpack-dev-server allow
"localhost" as well as its subdomains. As "localhost" is already
considered safe and convenient, the same can be said of its subdomains.

Available Workaround:

Without this feature, developing a multi-domain application requires
configuring webpack-dev-server with:

    allowedHosts: ".localhost"