Website Testing

I am studying IT and have a unit about testing websites before they are released. My task asks why testing is important and what problems should be checked. I know broken links and slow pages can cause trouble, but I am unsure what other examples to include. I noticed the phrase HNC assignment writing service while looking for study ideas. I want to write the answer myself, so I only need some direction. Should I focus on security, speed, user experience, or all three?