An id is defined as follows:
id = name [CS]
This attribute assigns a name to an element. This name must be unique in a document.
Then if you dig further you will see that a "name" token is defined as follows:
ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").
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