How can I prevent an XSS vulnerability?

Question

I going through the website when I realized that it's possible to do an XSS attack in HTML. I never really knew how to fix this. Does anyone have an idea?

Answer 1

This is a game / challange if you want to test it out XSS.
You will reward a cake if you completed :)

https://xss-game.appspot.com

Answer 2

Do not learn programming on sololearn.

Answer 3

If you accept user input, then you are vulnerable to XSS. To avoid this vulnerability use prepared statements or search for scripting tags in user input.