RSSOwl is lack of HTML support. So here is the fix this problem
Install mozilla-browser Using the following command
sudo apt-get install mozilla-browser
Change run.sh in your rssowl directory to:
java -Xmx134217728 -Djava.library.path=. -jar rssowl.jar
In RSSOwl, go to Tools-->Preferences-->View
In the "Misc" section, check "View newstext in browser"
Go to Tools-->Preferences-->Browser
In the "Please enter path to executable" box, enter the path to your exectuable (lol). This is the app that you want to use to open in an external browser, so I wanted to use firefox. My path was "/usr/bin/firefox" but that's not default. With a new system, it could be "/usr/bin/mozilla-firefox"
At the bottom, check "use external browser"
Now you should be able to view everything properly.