Hypertext Markup Language (HTML) and Cascading Style Sheets (CSS) are cornerstone of every website.
Whenever you open a web browsers like Mozilla-Firefox, Google-Chrome, Apple-Safari etc; they are primarily interpreting the HTML and CSS codes to display information from the website.
Learning HTML and CSS provides a platform for kids to express and exchange their ideas with the world.
Although, there are several other technologies/frameworks that add functionalities to a website, a perfect understanding of HTML and CSS is a prerequisite in good web design.