What's the deal with those bars?

[pardon]

Hi all,

Here's a questions I've been batting around: what's the deal with the 3px-10px solid 100% width bar at the top of websites? It's a trend that I've been noticing for a few years, usually on sites featuring longer articles / lots of scrolling, like these:

http://craigmod.com/
http://www.alistapart.com/
http://www.coudal.com/ (http://www.coudal.com/)(very subtle)
http://ilovetypography.com/
http://www.informationarchitects.jp/en/

Is there a UX purpose? Is there a browser-compatibility purpose?

I'd love to know the function and justification for using such a design element.

Thanks!