Prevent Some HTML tags to render on browser

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C#

MVC

MVC3

I have make a messaging system in which user can send messages to each other, they can also send files as attachement in message(its like simple email system). It allows users to send HTML characters and they'll render by browser, for eg if they enter

<b>Hello</b>

it'll rendered as Hello

Its working fine,however i am facing one problem if user enter

<iframe src="anywebsite"><iframe>
then it'll also rendered by browser as iframe

How can i allow only some particular characters to be rendered by browser rest will display as normal text I am using Asp.net MVC3

In my model class i've add
[AllowHtml] attribute to allow HTML characters

I've used HtmlSanitization Library[^] to render save HTML but in case of iframe it is rendering nothing and i want to display iframe as plain text