I am looking for answers if SharePoint online supports client-to-server authentication. My use case is to access Sharepoint REST API via pure JS with no server involved.
Mentioned below are few resources which might help you in understanding this method and implement in your solution.
You can also refer to the article from Andrew Connel on how to use ADAL JS with Angular to communicate with Sharepoint. ADAL JS & CORS with O365 APIs (Files & SharePoint)