With previous versions of ASP.NET MVC (including last
month’s ASP.NET MVC V3 beta) you needed to explicitly call
Html.EnableClientValidation() within your views in order to enable client-side
validation to take place. Starting with today’s RC that is no longer
required, and client-side validation (using an unobtrusive approach) is now
enabled by default (you can turn this off if you want through a config setting
in web.config).
You do still need to reference the appropriate jQuery+jQuery
Validation libraries within your site for the client-side validation to
light-up. Because you explicitly reference the JavaScript files, you can
choose to either host them on your own server or reference them from a CDN
(content delivery network) like Microsoft’s or Google’s.