Now with introduction of HTML5 you can use HTML5 validations in C # .NET webforms .
Validations like -Required field validations and Regular Expression validations are now simpler with HTML5.By Now all most all Browsers with there latest versions ,do support HTML5 tags and syntex.So you can rely upon HTML5 for client side validations.
Here is one of the best links i can find which will help You with basics of HTML5 Validations. How to use html5 validation in asp.net…. click here