About 15-20 years ago, web application developers had the knowledge and the technical skills necessary to create an application: HTML, CSS, JS, PHP/Python/ASP, Web Server Management, MySQL/Postgres (both data and service management), etc. 

With time the web applications became more complex with bigger requirements. New technologies, frameworks, and paradigms bounce into the development and pushed developers to specialize in some areas arising new roles from this specialization.

Source de l’article sur DZONE