nl_tokens: @param uri { string } Provider URI or Web ID URI
pl_tokens: function providerExists ( uri ) { let providerOrigin = ( new URL ( uri ) ) . origin let providerConfigUri = providerOrigin + '/.well-known/openid-configuration' return fetch ( providerConfigUri , { method : 'HEAD' } ) . then ( result => { if ( <mask> . ok ) { return providerOrigin } return null } ) }