Only fonts that are installed on the viewer's system will be visible. If they do not have that font then it just defaults to something else.
If you want to know what fonts are very likely to work, take a look at some
web-safe fonts. These are fonts that are practically accepted to be available on virtually every computer system, and therefore 'safe' to use on web pages. Obviously not Blockland specific, but good as a guide for choosing fonts that people are likely to be able to see.