How web Application works

There are so many paths that could be chosen here, there isn't a good answer. But we can break some of it down.

For sure, you need to show your user something, so at a bare minimum, you'll use html to structure content on the screen. These days, no one will really take your web app seriously without some unique branding and style, so we can almost guarantee you'll use css as well. It would also be very common, especially today to use javascript on the client side as well, although not quite as necessary as css or html.

But generally, to show our client something, you will have likely used html, css, and js on the client side. How to generator that code is up to you. There are many different back end languages that all net the same result. Php, ruby, python, node, and even more! And than you might use Sass, Less, or Styles to generate your css, and maybe you use coffeescript to generate your client side js. And you'll likely need to store data, so maybe you'll use a sql based database like the ever popular mysql, or postgresql, or another sql database option (yes, there's more!). Or maybe you decide to go the ways of a nosql database like something such as Mongo.

All this to say, nothing is short of html is "required" to build a web application. There are a lot of different ways you can go, and it's just a matter of fitting the pieces together to make an ecosystem of tools to build the web application.