When rendering a ContentArea with the CssClass property set, the class is removed when editing the page which breaks the layout. This code renders, as expected <ul class="slides"><li> etc when browsing the site.

@Html.PropertyFor(x => x.PromoArea, new { CssClass = "slides", CustomTag = "ul", ChildrenCustomTagName = "li" })

But in edit mode the class "slides" is replaced with "epi-editContainer".