The difference is that <%: automatically HTML encodes the string, while <% = not.
Back before MVC2 comes out, so that HTML encodes the string that you should have used in the presentation of the Html.Encode () method.
<%= Html.Encode(Model.MyString) %>
However, with MVC2 they added the <%: tag, which outputs the same, but handles the HTML encoding for you.
<%: Model.MyString %>
As a rule of thumb, you should always output your lines using the <%: tag, unless you have a good reason.
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ASP.NET 4 (<%:% > ) , , <% =% > do - HTML . HTML ...