HTML/CSS experts! Why do some sites not display correct fonts?

[juice]

I've been having a slightly annoying problem with certain websites not showing the correct fonts and defaulting to Arial.

This happens on a number of sites but I'll use Grooveshark as an example (attached). The headings should be in "Helvetica Neue Condensed/Helvetica/Arial/Sans Serif". I have all three of these fonts on my system however it shows Arial! (same on firefox, chrome & ie).

There are a number of other sites that use "non-standard" fonts where it works fine.

Any ideas?

 

[ThrashZone]

Couldn't do much with the site,
It's annoying insistence to use flash player was to much.
http://jigsaw.w3.org/css-validator/validator?profile=css3&warning=2&uri=http%3a%2f%2fgrooveshark.com%2f#!/

 

[A Guy]

Is this problem widespead, or limited to a few sites? Is it a problem that just started, or has always been that way. All 3 of your browsers can be set to display fonts (and colors) of your choosing if necessary.

https://support.mozilla.org/en-US/kb/change-fonts-and-colors-websites-use

https://support.google.com/chrome/answer/95416?hl=en

Change the colors and fonts used for webpages

A Guy