What's dp (density independent pixels) units with CSS?

I disagree with the currently accepted answer. As uber5001 suggests, a px is a fixed unit, and in a similar spirit to the efforts of the Android-specifc dp.

Per Material's spec:

When writing CSS, use px wherever dp or sp is stated. Dp only needs to be used in developing for Android.

Additionally

When designing for the web, replace dp with px (for pixel).

http://www.w3.org/TR/css3-values/#lengths

The closest unit available in CSS are the viewport-percentage units.

• vw - Equal to 1% of the width of the initial containing block.
• vh - Equal to 1% of the height of the initial containing block.
• vmin - Equal to the smaller of vw or vh.
• vmax - Equal to the larger of vw or vh.

The only mobile browser to be aware of that doesn't support these units is Opera. http://caniuse.com/#feat=viewport-units

Use rem.

It is the font size of the root element and a very good base unit for the size of other UI elements.

If one used the same absolute size (in centimeter), the text and other elements would be much too big on mobile or much too small on desktop.

If one used the same amount of pixels, the text and other elements would be much too small on mobile or much too big on desktop.

The rem unit is on spot because it says "Hey, this is how big normal text should be." Basing the size of other UI elements on this is a pretty reasonable choice.