If i understood you right, you would like to display string as HTML markup?
var ls = (List<string>)TempData["ModelLs"];
foreach (string str in ls)
{
@(new HtmlString(str))
}
Html.Raw()
works too :
var ls = (List<string>)TempData["ModelLs"];
foreach (string str in ls)
{
@(Html.Raw(str))
}