UI
This is actually not supposed to be a public web page.
The http://omnifaces.org/ui is supposed to be the XML namespace URI of OmniFaces UI components.
Usually, you declare this URI in the XML root element of your XHTML file as below in order to use OmniFaces UI components within the element.
<html ... xmlns:o="http://omnifaces.org/ui">
Normally, XML namespace URIs do not represent actual web resources.
That they start with http:// is indeed confusing to starters, but that's so far the best way to have a unique resource identifier (URI) for a XML namespace.
How does it work then?
During webapp's startup, Facelets will parse any .taglib.xml file found in the web application and remember its structure by the <namespace> entry.
This way Facelets can find the appropriate tagfiles, TagHandler classes and/or UIComponent classes for any <o:xxx> tag.
Also, this way any IDE such as Eclipse can load the appropriate tag/attribute listing/documentation which is displayed on mouseover or autocomplete.
You can find the OmniFaces UI .taglib.xml source code here.
But my OmniFaces tags doesn't work!
And that's why you looked up here?
Well, apparently the OmniFaces JAR is just missing in webapp's runtime classpath.
Make sure that the JAR file is present in /WEB-INF/lib folder of the built WAR file.
If you're not using a dependency management framework such as Maven,
then this is usually to be achieved by manually placing a copy of the JAR file straight in the /WEB-INF/lib folder of your webapp project.
Got it
Go back to OmniFaces homepage then :)