These are the five technologies you must master if you are thinking about web development as a career and you have opted to go by the open source way. With plain simple HTML (HTML 5 - the current version)you can create static webpages and have text, images, videos, audio on those pages. If you want your pages to look good CSS is required and if you want to add dynamic functionality to the page Javascript becomes necessary. If your page adds and retrieves information to and from a database and if you want to perform some calculations and if...else type functions then PHP and SQL are required. In short, for simple sites HTML+CSS is sufficient. As the site becomes complex, and that is inevitable, PHP and SQL are required.
All the above mentioned technologies are free and open source.
MDN Web Docs - Mozilla (The Firefox company)
These are the two excellent sites to start learning all the above mentioned technologies.
I have created some sample pages - you may go through the page source.
Jquery, Bootstrap and similar Javascript libraries simplify using Javascript. They help to reduce the amount of coding and thereby the time taken to develop. I have made use of jquery and bootstrap in some pages.
List of directories and files in this folder