HTML5 is the latest version of the HyperText Markup Language used for defining webpage content you view with a web browser. It is rare for developers to be writing webpages by hand these days, user interface frameworks like Vue, React and Angular are popular for giving each page real time responsiveness that users have come to expect from web applications. These interfaces can be served by almost any backend, from Python to PHP on web based servers, to mobile apps created with Cordova for Android and Apple iOS devices. Server side CMS's like Python's Mezzanine and PHP based CraftCMS can allow almost anyone to create modern webpages without any development experience.
HTML gives you an easy, well documented way to design content and user interfaces across almost any modern device. You can find developers almost anywhere that can quickly and easily implement striking designs crafted by design agencies for almost any purpose. From marketing to modernizing access to legacy systems, HTML is at the heart of almost every modern application.