In Web design, sizes can either be expressed in absolute or relative terms. Absolute value units are used to precisely control sizes, so that they do not vary regardless of the particular screen size, browser, or operating system being used. That said, even absolute units can vary between computers, generally because of the operating system. This was the case with point sizes between Mac and Windows.
Relative value units have no fixed size, but instead are calculated relative to another value, such as the parent elements size or to the screen itself. Although less precise, relative values have the advantage for screen that they can be quickly scaled and changed without having to recalculate all of their dependent values. For example, if you are using relative values to set the font size and line height, simply changing the font size will also change the line height proportionally.
Although it is possible to precisely control the position of elements with any of the absolute units, pixels are the most natural way to define measurements for screen based media. Despite being a relative size, pixels behave absolutely in context to the screen resolution, and many modern Web designs will be specified in pixels because it is the most universal measurement regardless of screen size, OS, or browser. Like atoms in molecules, pixels are irreducible as the smallest unit on the screen—you can’t move something half a pixel.(http://www.peachpit.com/blogs/blog.aspx?uk=Pixels-or-Ems-in-Your-Web-Designs)
Most Web browsers allow users to enlarge text and zoom the page size, which is imperative for anyone with poor vision. Locking the font size with pixels or absolute values prevents Internet Explorer from changing their size. Internet Explorer 7 goes some way in rectifying this limitation by allowing for the entire page to be zoomed, but there is still some debate over their use when accessibility is an issue. So, by using pixels you get precision in your design at the sacrifice of versatility.In order to provide the reader with highest level of control over the content they are viewing, it is increasingly considered a best practice to define sizes using ems.The em is the fundamental unit of measurement in typography, defined as the size of the type as computed relative to the current size of the type of the parent element.
|pt||point||72pt = 1inch||12pt|
|pc||pica||1pc = 12pt||1pc|
|mm||millimeter||1mm = .24pc||4.17mm|
|cm||centimeter||1cm = 10mm||.42cm|
|in||inch||1in = 2.54cm||.17in|