基于其他表单输入和验证显示/隐藏表单元素的最佳方法？

I am in the process of revamping a website using ASP.NET C# web forms. I have very little experience on the matter, and wondered. Is there best practice for showing or hiding form elements based on what is selected on other elements in the form in ASP.NET using c#? For example, if I select option 1 from a drop down list, then an additional text box appears. Right now I am using jQuery to accomplish this. Which leads me to my second question as I am not sure how I will validate. Is there a best/preferred practice for validating form fields in ASP.NET? I have seen validation using knockout.js, ASP:RequiredValidation control, etc, but would like to know general consensus. I am in the process of revamping a website usin