The native request validation is done via the native ASP.NET WebForms request validation and its handling is entirely up to the application configurations. More about this is available in this MSDN article
If by html you mean the content in the RadEditor, then this is habdled by the RadEditor entirely via client-side techniques. The RadEditor renders a hidden textarea element that preserve the content through post backs. The html content that is inserted is encoded and preserved on client-side via the submit event and on pageLoad is decoded and retrieved from the hidden textarea.
Further, the value of the hidden textarea is handled using the ViewState, which is again entirely handled by the native ASP.NET technology.
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