Javascript.
Then of course commercial, social, political, and many other areas jumped on board. People found out that sales (if you was commercial), views (if you were political or social), and whatever you were aiming for essentially, magically increased if you made your website _look_ better. It's how humans have always worked.
This gave birth to an arms race where the weapon of choice was including increasingly advanced ECMAScript code into your website.
From this the modern web was born. (i'm simplifying a lot here, but that is generally the very quick overview as to how I experienced it over the last 2 decades).