Do a publishMavenPublicationToMavenLocal on the plugin project before running the web project
main.gsp from web project that includes a foo.css from the plugin's assets dir.
<!doctype html>
<html lang="en" class="no-js">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<title><g:layoutTitle default="Grails"/></title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<asset:stylesheet src="application.css"/>
<asset:javascript src="application.js"/>
<asset:stylesheet src="foo.css"/>
<g:layoutHead/>
</head>
<body>
<div id="grailsLogo" role="banner"><a href="http://grails.org"><asset:image src="grails_logo.png" alt="Grails"/></a></div>
<g:layoutBody/>
<div class="footer" role="contentinfo"></div>
<div id="spinner" class="spinner" style="display:none;"><g:message code="spinner.alt" default="Loading…"/></div>
</body>
</html>
