I'm not able to replicate this on your site. I can see the <style>0</style> in one article, but if I remove it and save, it isn't added again. Also, editing and saving other articles does not add a <style>0</style> to the article.
Just because you're not paranoid doesn't mean everybody isn't out to get you.
OK! Then I apologize.
I was able to localize the problem with me, it is here at a browser extension that apparently provides for the problem, if I at least Deaktiviere the problem no longer occurs with me.