Google XSS challenge: Level 1 aka Hello world of XSS (detailed walkthrough)
Prerequisite Before getting started one should be familiar with XSS or at least have an idea about it. Here is a good article which you may give a read to understand what is XSS. Read! Also, I assume that readers are at least familiar with JavaScript. If not then I’ll suggest to spend some time with JS and get comfortable with the basics. You can refer to javascript.info and MDN which are extremely helpful....