Why isn the JSP editor/validator finding my custom tag libraries?
If your project’s Web Module facet version is set to 2.2 or lower, you will only be able to refer to libaries by their filename or mapped URIs in your web.xml file. For Dynamic Web Projects, the tag libraries must be either on the Java Build Path or under the WebContent folder. Otherwise, whichever parent folder is also the parent of WEB-INF is treated as the root of the web application, unless your project has the Module Core nature set up, in which case that information is used. For “well known” tag libraries that may be incorporated within a server runtime and not otherwise found, the XML Catalog may be used to register URIs for a specified TLD file on disk.
