[thoughtpalette]

"Nine out of ten times I want to do something with CSS I prefer doing it with JavaScript"

You're doing something wrong then.

[debt]

center a div both vertically and horizontally to the viewport; also it should stay center when resized.

[thoughtpalette]

Absolute Horizontal And Vertical Centering In CSS

http://coding.smashingmagazine.com/2013/08/09/absolute-horiz...

[debt]

Truth. The div would need a variable height though which would change the design.

[proexploit]

The page linked includes techniques for both variable height content (e.g. translate -50%) and fixed content.

[ptx]

Although this used to be a problem with CSS, it has been fixed in modern browsers: http://philipwalton.github.io/solved-by-flexbox/demos/vertic...

[pjmlp]

Now make it work across all browsers and operating systems required by your customer.

[elclanrs]

You can use viewport units in modern browsers. See here http://caniuse.com/viewport-units