Is also a possibility. There are a lot of different fonts but I don't know whether there's exactly Arial Rounded MT Bold. You can use only fonts which are installed at the clients computer!! So just mentioning them in the css font-family doesn't work if the font is not installed.
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For this, Google Fonts and others bring the functionality. So everytime you use a font that is not 'Times', 'Arial' or 'Verdana', which are installed on nearly every computer, you should think about an alternative to raw css.
There's a good chance that — if nothing changed (on your local browser's rendering of the webpage) when you changed the font stack — that you don't actually have Arial MT Rounded Bold installed on your computer. One way to test this is to use Firebug, to 'inspect element' for the headline (or whatever the font is applied to), and to edit the font-family listing in the right-hand Firebug pane. (Usually, I just find the named font and add a space into the name of it, which 'breaks' the call to the proper font, and the browser then moves on to the next font in the stack.) Other options include using Google's web-font Nunito (as Kent Brewster recommended a few minutes ago), or setting up a font-face font on your own server, providing one of the rounded fonts (RockoFLF Bold, FF Din Rounded, Nunito, and Arial MT Rounded Bold are all options). (Technically, per Kent's comment, Glitch is using RockoFLF, with Arial MT Rounded Bold as the second font in the stack. At least, that's what they were doing a few months ago when I last looked at their code. Just checked again, and they've dropped the Arial MT Rounded Bold out of the stack. They're calling RockoFLF with an embedded font-face.) Anyway, as Kent Brewster noted, I think your best bet is to use Google Web Fonts' 'Nunito', if you aren't sure that it'll be on your users' computers (or even your own).