Localhost Security Messaging
Browsers these days either mark sites with a padlock ( https://) or "not secure" ( http://). This warns the users that without the protection of " https://" your communications could be read or modified by any network your packets travel over. But how should " http://localhost" be marked? That's your own computer so it's secure, but the connection isn't encrypted so a padlock would be misleading.
It turns out that the browsers have three options for the url bar, not just secure and insecure. Here's what they look like in Firefox:
Chrome:
Safari:
Despite the unusual URL bar treatment, the major browsers do now all treat this configuration as a secure context (spec), which means you can use features that require secure contexts, like crypto, MIDI, or geolocation.