The API at address https://api.mojang.com/users/profiles/minecraft/<username> doesn't respond with CORS headers, particularly Access-Control-Allow-Origin: * is missing. Without this header modern browsers will discard the response and create an error message.
In the Developer Tools of Firefox this looks like:
fetch('https://api.mojang.com/users/profiles/minecraft/test').then(console.log)
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://api.mojang.com/users/profiles/minecraft/test. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).
TypeError: NetworkError when attempting to fetch resource.
To set mode:no-cors is not an option because in this case the browsers exchange the response with an opaque one, which is basically empty.
fetch('https://api.mojang.com/users/profiles/minecraft/test', {mode:'no-cors'}).then(console.log)
Currently the only way to use this API with browsers is to set up an additional server as a proxy that adds this header. But that's not what I want. In times of GDPR I'm not interested to run such a server or to use a third party server for a web application that should only run in the browser. Please consider to add the mentioned header.
For the records, more than two month after this ticket was set to "Resolved" the problem still exists.