PHP is open source and the most widely used web backend language, by far. As is Python and Node.JS, both also open source and very popular. Actually, most major programming languages are open source.
For frontend, angular, react, vue, and jQuery are also open source, and last but certainly not least, Bootstrap.
Apache and Nginx web servers have a huge combined market share, both are open source. As are popular databases like MySQL, PostgreSQL, MongoDB, etc. As are other web software like Redis, Docker, Kubernetes, etc.
The vast majority of websites use GNU/Linux servers, which are also open source.
For specific examples that I can think of, Instagram is written in Django, which is an open source web framework in Python. Reddit is written in Pyramid, another open source Python web framework. In fact all of Reddit was open source until a few years ago.
Depends on what you mean by “built with”.
PHP is open source and the most widely used web backend language, by far. As is Python and Node.JS, both also open source and very popular. Actually, most major programming languages are open source.
For frontend, angular, react, vue, and jQuery are also open source, and last but certainly not least, Bootstrap.
Apache and Nginx web servers have a huge combined market share, both are open source. As are popular databases like MySQL, PostgreSQL, MongoDB, etc. As are other web software like Redis, Docker, Kubernetes, etc.
The vast majority of websites use GNU/Linux servers, which are also open source.
For specific examples that I can think of, Instagram is written in Django, which is an open source web framework in Python. Reddit is written in Pyramid, another open source Python web framework. In fact all of Reddit was open source until a few years ago.