The white paper briefly discusses the CssSelectorClass expando property:

http://www.asp.net/CSSAdapters/WhitePaper.aspx#SamplesCssSelectorClass

The key here is to remember that this is an expando property.  That is, it is a property that we've suddenly invented for use by the adpaters; the rest of the framework is supposed to (and does) simply ignore this extra server-side property.

Because CssSelectorClass is an expando that we've invented, it isn't known or recognized automatically by tools like Visual Studio.  They see it and think: hmmmm, maybe there's a typo in the markup. That's why it is a warning, not an error.  VS can't be sure whether or not you intended to include an attribute (CssSelectorClass in our case) that it can't seem to find in the normal set of attributes allowed for the Menu tag (or the DataList tag or the TreeView tag, etc.).

 Hi, I understand that this is a workaround-feature, but Publish fails with the same error. How can i get Visual Studio 2005 to ignore these properties? Thank you.