The [font] tag allows you to assign any font to your text and it works independent of the forum software. Basically, it doesn't matter if the specified font is installed on the server machine or in the supported fonts list, once you assign a font to your text using that tag those with that font installed on their system will be able to view it in the assigned font, otherwise it's displayed in the default forum font. The posts you've linked are all in fixedsys so if it's not displaying them with that font for you, it means the issue is on your end and you may want to reinstall the font, that should fix the problem.