Here is the start of my JSF file:
<?xml version='1.0' encoding='UTF-8' ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:h="http://java.sun.com/jsf/html"
xmlns:ace="http://www.icefaces.org/icefaces/components"
xmlns:c="http://java.sun.com/jsp/jstl/core"
xmlns:f="http://java.sun.com/jsf/core">
<h:head>
</h:head>
<h:body>
However, http://www.icefaces.org/icefaces/components and http://java.sun.com/jsp/jstl/core are highlighted in red with error URI is not registered. This causes many other subsequent tags to appear in red with errors element 'x' is not allowed here. Here is an image showing what I mean:

Attempted solutions:
When searching for a solution, it appeared some others with the same problem solved it by doing "Fetch External Resource". However, this didn't work for me. For the icefaces xmlns xmlns:ace="http://www.icefaces.org/icefaces/components" I get the error: Error while fetching http://www.icefaces.org/icefaces/components. And for the jstl xmlns xmlns:c="http://java.sun.com/jsp/jstl/core" I get the error: No XML at the location: http://java.sun.com/jsp/jstl/core.
I also found some people solved it by deleting the IntelliJ IDEA metadata file workspace.xml in the .idea directory. This does seem to work temporarily... but then it seems the problem comes back after a while. I also have to reconfigure several project settings which I would like to avoid.
Any help would be appreciated.. I'm not really familiar with the xmlns stuff and still a beginner to web app development. Thank you!
Update:
As per Mark Vedder's answer, I manually added external resources to my project and the problem has not resurfaced thus far.