Use only lower case (a–z) or upper case (A–Z) letters, numbers (0–9) or one of the symbols; $ (dollar sign), - (minus), + (plus), or _ (underscore) for an ID. This ensures that your ID can be used in all major environments, e.g. HTML, XML and SGML. Other characters and symbols, e.g. spaces, may result in your ID not being accepted in these environments.