The id global attribute defines an identifier (ID) which must be unique in the whole document. Its purpose is to identify the element when linking (using a fragment identifier), scripting, or styling (with CSS).

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:

• CSS ID selectors accept any CSS identifier. If the id starts with a digit or one hyphen immediately followed by a digit, both the hyphen and digit must be escaped in CSS. For example, while id="544-383-3388" and id="-3Pi" are valid in HTML, the id selectors must be escaped. The element with these id values can be targeted in CSS with #\35 44-383-3388 and #\2D \33 pi.
• Any valid HTML id value is valid as an attribute selector in CSS and JavaScript. The selectors [id="544-383-3388"] and [id="-3Pi"] are valid.
• Document.querySelector() and similar methods using CSS-selector-style queries will not find them unless you escape them. (See this page for an example.)
• Such an id is not a valid JavaScript identifier. Elements with IDs become global properties, but you cannot use non-identifier global properties as global variables — 1234 is not a global variable, and you have to use window[1234] instead to get the element with id="1234". This is slightly inconvenient as you have to create the variable with an extra step: const element = window[1234].

• All global attributes.
• Element.id that reflects this attribute.
• The Document.getElementById method.
• CSS ID selectors.