Cm stands for CSS centimeters. Similar to other CSS absolute units, centimeters map directly to pixels, with 1cm=96/2.54px or approximately 37.7px.

There's a lot of content created with HTML and CSS that isn't meant for the web. For example, CSS is widely used to format printed books and ebooks because HTML & CSS are handy formats to standardize and are far easier to deal with than MS Word or a desktop publishing package.