Warning: This attribute's value is an opaque string: this means that web authors should not rely on it to convey human-readable information (although having your IDs somewhat human-readable can be useful for code comprehension, e.g. consider ticket-18659 versus r45tgfe-freds&$@).

An id's value must not contain whitespace (spaces, tabs, etc.). Browsers treat non-conforming IDs that contain whitespace as if the whitespace is part of the ID. In contrast to the class attribute, which allows space-separated values, elements can only have one single ID value.

Technically, the value for an id attribute may contain any character, except whitespace characters. However, to avoid inadvertent errors, only ASCII letters, digits, '_', and '-' should be used, and the value for an id attribute should start with a letter.

For example, . has a special meaning in CSS (it starts a class selector). While valid, unless you are careful to escape it when used as part of a CSS selector, it won't be recognized as part of the element's id. The same applies to the querySelector() and querySelectorAll() parameters, which use the same selector syntax. It is easy to forget to do this, resulting in bugs in your code that could be hard to detect.

Similarly, an id starting with a digit (E.g., 1234-322-678) or a hyphen followed by a digit (E.g., -123), though valid in HTML, may lead to problems when used in CSS, JavaScript, and Web APIs: