I haven't tried this, but you can try the DesignMode Property:
Private Sub WebBrowser1_DocumentCompleted(ByVal sender As System.Object, ByVal e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) Handles WebBrowser1.DocumentCompleted
obj = WebBrowser1.ActiveXInstance
obj.document.designmode = "On"
Application.DoEvents()
End Sub
'To Bold Text
WebBrowser1.Document.ExecCommand("Bold", False, Nothing)
'To Underline Text
WebBrowser1.Document.ExecCommand("Underline", False, Nothing)
'To Italisize Text
WebBrowser1.Document.ExecCommand("Italic", False, Nothing)
'To Set Text Font
WebBrowser1.Document.ExecCommand("FontName", False, "Calibri")
'To Set Text Font color
WebBrowser1.Document.ExecCommand("ForeColor", False, Color.Red)
'To Set Text Back Color
WebBrowser1.Document.ExecCommand("BackColor", False, ColorDialog1.Color)
'To Align Left
WebBrowser1.Document.ExecCommand("JustifyLeft", False, Nothing)
'To Centre Align
WebBrowser1.Document.ExecCommand("JustifyCenter", False, Nothing)
'To Right Align
WebBrowser1.Document.ExecCommand("JustifyRight", False, Nothing)