I'm guessing it's done as an experiment and proof-of-concept instead of being offered as a best practice.

yes it is done as an experiment but don't you think by using CSS3 ... you don’t have to regenerate images every time you change the text, pages will load much faster because you don’t have to download separate images . i can say by css3 in some portion we can replace the images.

The browser has to render the CSS, which takes a non-zero amount of time. It's possible that the browser rendering time of some (very) complex CSS could surpass the time to fetch an image over the network and render that.

Some of us cannot make a nice looking shiny "Sign Up" button in Photoshop. So this is great.

Errr, isn't that what PNG is for? o.O

the size of PNG is much bigger than one css file .. isn't it ..

Also, gzip is much more efficient on plain text.

very much true..