Are websites hackable?

Question

So as the title you can see that my question is the same--
"Are websites hackable? If so is it possible even someone uses simple HTML,CSS and JavaScript? Also how to prevent it from happening?"

Answer 1

can be safe, just give a proper rules, handle every exceptions. (including variables, visibility)
if you have form, use the method post, but validate all data both in frontend and backend, etc