The ID Attribute

I wanted to use the id attribute to make anchors for a FAQ using numbers 
like 01, 02, etc. But if the value of id is numbers only, it isn't valid.
Here's an example:

<span id="01">Question 1</span> This is invalid while <span 
id="question01">Question 1</span> is valid.

Why is it invalid just because the value is numbers only?