The JSP validation supports and understands JSP 2.0 tag files. However, the in-editor annotations don’t yet have support for these files. So the <tags:greetings /> tag will show up as an unknown tag, but when you save the file, the jsp validator will ensure the greetings.tag file exists.
I’ll check with the dev team on the status of JSP 2.0 tag file support in the editor.