MVC will automatically protect your application from some html injection and cross-site scripting (XSS) attacks. This is why you will get the "A potentially dangerous Request.Form value was detected from the client (...)" by default when attempting to post html/javascript.
However, we may sometimes want to allow our users to post html. You might just want to allow users to use characters such as "›", or it might be because your implementing blog functionality and want to support tags like ‹h1›, ‹div›, etc. This can easily be accomplished with MVC by disabling request validation.
Add [ValidateInput(false)] attribute to the action method in the controller you are calling. This will disable request validation for the entire model on the specific action.
Another way is to add the [AllowHtml] attribute to the property which requires html in your model.
These two attributes will only allow html/javascript to GET IN to your application, but MVC will still output them safely by using html encoding. If you want to output it like html, you can use the @Html.Raw(@Model.Content). But use this with caution, since this will open your application to cross-site scripting attacks (XSS)!