The WSO2 Application Server 5.3.0 introduces key features including Java EE 6 web profile support, Tomcat virtual host support, and WebSocket 1.1 API support, enhancing its capabilities for web applications. The release includes a shift to GitHub for continuous integration and a drop of Java 6 and default data services hosting support. Additionally, it features SAML2 SSO support for web applications and is built on the WSO2 Carbon middleware platform.