What values are valid for an ID attribute in an HTML element?

When naming id attributes of HTML tags, you do have some limits.

As stated by the HTML4 specification...

>> 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 (".").

Source: https://www.w3.org/TR/html4/types.html#type-id

HTML5 has more-relaxed specifications...

There are no other restrictions on what form an ID can take; in particular, IDs can consist of just digits, start with a digit, start with an underscore, consist of just punctuation, etc.

Source: https://www.w3.org/TR/html5/dom.html#element-attrdef-global-id