Andrew,
You need to think of the browser as a translating program - it translates from the arcane languages of HTML and CSS into something that (hopefully) looks pretty and is intelligible.
But consider two humans whose jobs are to translate Russian to English. Their "output" would both mean the same thing, but the actual words used, accent and inflection could be very different.
Browsers are just the same.
In this case, the CSS code requests that the "scrollbar colour" should be a particular shade of red. Now, some older browsers don't know about CSS at all, so will ignore my request. Others, like Safari, will understand CSS, but they were never taught the particular word "scrollbar-color", so they'll pay attention to other requests, but ignore that one. The ability to change the way the scroll bar looks is something invented, not by the lovely people writing stanards at the IETF (Internet Engineering Task Force) or the W3C (World-wide-web consortium), but by... Microsoft.
Microsoft's plans for world domination include tempting designers to use "cool" features that only render as the designer intended in Internet Explorer. Eventually, pages may contain so much IE-specific stuff that they'll not render intelligibly in other browsers, so the users migrate to IE, and Bill Gates is happy. And the remainder of us are not.
Fortunately, the site has been designed to "gracefully degrade" on browsers which do not support all the features available, so everyone stays happy
Hehe!
