html - Why can't I have a numeric value as the ID of an element?

Question

Working on a project, nearly finished and just tidying up the HTML and I find out that you're not really allowed to have an ID that is just a number-

< a> attribute "id" has invalid value "567" The attribute ID is of type ID. As described above, it should begin with a letter and have no spaces

Good    <a id="567" href="/index.html">
Good    <a id="n567" href="/index.html">

I can go through my code and add a letter then strip it when the value is used in my jQuery but it would be messing around I don't really need.

Is there a reason I shouldn't be using numbers as ID's?

Answer 1

That's just what the spec says.

From the HTML 4 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 (".").

The good news is that the HTML 5 specification is more lenient:

The id attribute specifies its element's unique identifier (ID). The value must be unique amongst all the IDs in the element's home subtree and must contain at least one character. The value must not contain any space characters.