Generally speaking, characters not found in the normal alphanumeric set must be added to a web page by way of character entities. These take the form
n being a two- to four-digit number. Many entities also have a name, which tends to be more convenient and memorable; these are also listed. However, entities are case sensitive, so take care when adding them to your web pages.
Although most browsers display nonalphanumeric characters when the relevant encoding is specified, it's sometimes necessary to use entities to ensure your page displays as intended across a large range of machines.
Most reference guides tend to list entities in numerical order, but I find it more useful to browse by grouped items, so ...