Font definition format?

I was looking at a site and i noticed in the status bar of IE5 a message -
'Downloading Data .../fonts/
Being curious i had a look at the source of the offending page and this is
what i found:
<LINK rel="fontdef" src="fonts/futuraLtBT.pfr">

Now this seems a rather useful tool. The font they were displaying on the
page, was not one that I had on my computer before - and still havn't
according to the fonts folder in Win98.
I remember someone saying that there was a new 'protocol' coming along that
would automatically download and install a font if it were not present on a
user's system.
Is this it?
Surely this is not cross-platform stuff - I shall have a look in Linux in a
minute.
For those who are curious, the site was www.thus.co.uk

Any ideas?

Ben Bradley UK
Creative Director

INTERNET: http://www.thenetzone.co.uk
FAX: 07080 810966 (From UK)
PHONE: 07941 120956 (From UK)

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