Letter spacing is meant to provide an even space between letters, making it easier for web visitors to comprehend the content of your website design. It has a decisive influence on the readability of the text. If the letter-spacing is too large or too small, the legibility of the text will be affected. The words may not be correctly comprehended and this could lead to a lot of mistakes or, the reader will be forced to spell out the text (which is even more annoying).
The general rule is this: the smaller the text, the larger the letter-spacing must be. In lead typesetting, all letters used had a fixed size and were equipped with a corresponding spacing. Digital fonts are different. They can be set in any size and typographers typically have the power to adjust them according to the demand of the current trends. For example, digital fonts are aimed at a size between 10 and 16 points. For the other sizes, the typographer must make some light corrections to fit them for web use.
On the web, the minimum raster on which the letters can be arranged has a size of just one pixel. But in most cases, a 1-pixel spacing can be too much to make proper corrections to the text. While most web designers use this spacing for the body of the text, the headers and subheaders are differently designed. In headlines with a size of 24 points or more, you can use pixels to correctly space the letters. A header with a line spacing of 3px, for example, is easier to read and understand on your website design.
It’s a little bit tricky to maximize letter spacing, though, because those that use 5 pixels and above run the risk of being unreadable. Too much space is bad, too. It can be problematic to differentiate the words from each other because the space between letters and between words will look similar.
Then, there’s also the issue of inversely-set text, meaning bright letters on dark background. There’s a separate rule for this as well. When the text is set like this, it tends to appear overexposed. The letters must have a bit more spacing compared to the spaces we use when we post the text on a lighter background.
You can always use the font-size dependent unit “em” for letter-space corrections. This will ensure that the letter-spacing will change and adjust along with a change to the font size in the browser.