Why do only some of my system fonts work in my HTML?
Using anything but standard-fonts is possibly only since very recently: http://www.tudy.ro/2008/09/02/embedded-web-fonts-are-back/.
It won't work in all browsers, though.
This might be interesting, too: Typekit
DIN Engschrift is available in OTF, PS, and FF formats, for both the Mac and PC. It's possible that the browser you are using does not fully support the font format you have installed. Try adding the other formats, or try a different browser.
I assume you already realize that any HTML pages you make with unusual font5s will not render the same on a machine that lacks those fonts.
Supported fonts depend on the browser, though Firefox on my Mac apparently allows me to use any font available on my computer. However, since I am a Web developer and so I expect people who have different OSes to see my site, I am only using fonts that are "Windows/Mac Web-safe" (Mac OS X helpfully lists those as a collection in Font Book)
This isn't much, but world isn't ideal.
It's possible with CSS Fonts module to tell the browser to load a font you want (@font command), but the practical support isn't great.