That would be nice if <tag> declaration from *.taglib.xml are discovered and used in VPE content assists.

Here is scenario for facelets taglib registered in web.xml:
1. EXECUTE: Create JFP Project using New JSF Project Wizard
2. EXECUTE: Enter a name, select JSF environment: ['JSF 1.2 with Facelets', select Template: 'FaceletKikStartWithoutLibs' then press 'Next'
3. EXECUTE: Select Server Runtime and Target Server then press 'Finish'
4. EXECUTE: Copy attached files custom.taglib.xml and inputCurrency.xhtml to /WEB-INF
5. EXECUTE: Copy attached file show100.xhtml to WebContent/pages
6. EXECUTE: Open web.xml in any editor and add text below
<context-param>
<param-name>facelets.LIBRARIES</param-name>
<param-value>/WEB-INF/custom.taglib.xml</param-value>
</context-param>
8. EXECUTE: Open show100.xhtml in VPE
7. ASSERT: Content proposal list for <c: should contain inputCurrency tag
9. ASSERT: Ctrl+click on 'xmlns:c="http://mydomain.com/jsf/myproject"' opens custom.taglib.xml in xml editor

Scenario for facelets taglib in jar is almost the same but .taglib.xml should be discovered in <filename>.jar/META-INF/.taglib.xml.