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).
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
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.
Note: Technically, in HTML5, the value for an
id attribute may contain any character, except whitespace characters. However, to avoid inadvertent errors, only ASCII letters, digits,
'-' should be used and the value for an
id attribute should start with a letter. For example,
. has a special meaning in CSS (it acts as a class selector). Unless you are careful to escape it in the CSS, it won't be recognized as part of the value of an
id attribute. It is easy to forget to do this, resulting in bugs in your code that could be hard to detect.
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