Server based WebApplications? now (2004) need to attend to increasingly sophisticated attacks due to CrossSiteScriptingExposure.
XSS is the acronym coined by security practioners to distinguish CrossSiteScripting from CSS which is the term for CascadingStyleSheets.
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MicrosoftDotNet concerns
MicrosoftChannelNine has this post for code related to AspDotNet at http://channel9.msdn.com/wiki/default.aspx/Channel9.HowToPreventCrossSiteScripting