The more exact and precise that your web style guide is, the more money and time you’ll save in the long run. You should detail the guidelines as closely as possible: Colors should be clearly defined with hex codes, RGB values, and associated mapping, for example. A short description can also be useful to explain why the given color(s) were chosen and the effect that they’re supposed to have on the viewer. It’s also crucial to outline which colors have been chosen for which element. You’ll also need to specify where to place icons, button elements, banners, and logos, as well as determining their colors of course.
All this information means it can sometimes be easier to offer a graphical overview rather than just a descriptive text. This graphic can also contain specific information about column width and line height etc. In doing so, the designer is given direct instructions about how all elements should appear on the finished page. Sizes for images, icons, and logos are also useful.
Font choice, size, and purpose of use should also be defined in your web style guide: how, when, and where should particular fonts be used? A good style guide will also ideally include direct examples for usage (designed to clarify which font should be used for which section, heading size, etc.). It’s also important to include information about line spacing, indentation, and distances between individual page elements.