I only need the Impact font for text in my site's header.
It looks like you've already got that set up correctly. Defining a font for some specified purpose, such as headers, while leaving it undefined elsewhere, will never cause the named font to spill over where it isn't wanted.
:: pause to shudder at the idea of body text in Impact ::
The section on @font-face at at w3.org seems be be written for the sole purpose of scaring people out of their wits, so let's paraphrase (here cribbing from one of my own directories)
h1 {font-family: Impact;}
@font-face {font-family: Impact; font-weight: normal; font-style: normal; src: local("Impact"), url("/fonts/Impact.ttf");}
@font-face {font-family: FreeHeader; font-weight: bold; font-style: normal; src: local("Impact"), url("/fonts/Impact.ttf");}
The part with "local" is optional in an @font-face rule; it's most appropriate when dealing with a font that many users do have installed, so only the others will need to download. If you're unfamiliar with the wording of an @font-face line, note how it behaves:
“When the code specifies a normal-weight, non-italic Impact, get it from suchandsuch source.”
The second @font-face line, which looks exactly the same, is because the Impact font doesn't actually come in bold (or, for that matter, italic). If you goofed and the browser thinks it's supposed to be using a bold version--this is especially likely in headers, because their default is bold unless your CSS specifically says "font-weight: normal"--you protect yourself by telling the browser to use
the same file. Otherwise it will try to fake it from the non-bold version; as you can imagine, this would look pretty ghastly in Impact. Italic would be even worse, but italic doesn't sneak in by accident. Add another pair of @font-face rules if you really want to play it safe.
And then, of course, you need your .ttf file installed where the CSS says to find it.
:: detour to check ::
On my system, the size of Impact.ttf is 138K. That's small enough to be embedded as-is. If it were larger than around 200K I'd edit it down in a font editor so users on smartphones aren't forced to download a bunch of stuff they will never use. (This is most dramatic in Chinese/Kanji fonts, which can run to tens of megabytes.)
:: further detour to FontBook ::
You may only (i) embed this font in content as permitted by the embedding restrictions included in this font; and (ii) temporarily download this font to a printer or other output device to help print content.
And, finally ... By the usual yawn-provoking coincidence, there have been a couple of threads about font naming just within the last couple of days. I think both were in this very subforum (CSS) so go read them. Option B is, as always:
h1 {font-family: Impact, sans-serif;}
There used to exist a splendid website that gave the percentages for various fonts on various operating systems, so you know which ones are safe to use, but it seems to have gone under :(
