HTML (Hypertext Markup Language) is the set of markup symbols or codes inserted in a file intended for display on a World Wide Web browser page. The markup tells the Web browser how to display a Web page's words and images for the user.
Why is is important to master this?
Many developers rely heavily on frameworks such as Ionic, Bootstrap and Foundation to create clean and elaborate layouts. Unfortunately, this usually results in over-use of markup for something specific making for a bulky structure that is not maintained very easily.