using 100%

i have seen in many websites they use
html { height: 100%; }

i normally do not use this what does html { height: 100%; } actually mean is this more of an optional thing or if i use this what significant advantage will it have for the pages i create.

please advice.

thanks.

It allows elements to have 100% height (i.e an overlay covering the full screen - lightbox for example)

Although I generally set it as:

Yeah, it usually makes more sense structure-wise to add the attribute to the body tag rather than addressing the entire HTML document.

Lee's answer is right.