I was recently sent this question:
I am trying to apply a div to a page and it will not show when I use the name “420wide” but it will if I use “wide420”. Why is this? Is this rule documented somewhere?
The simple answer is that the ID name must be unique in a document and it must begin with a letter – not a number.
You can find out more in the W3C’s HTML specification relating to the id attribute:
id = name [CS]
This attribute assigns a name to an element. This name must be unique in a document.
There are two associated links with this info:
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 (“.”).
CS – The value is case-sensitive (i.e., user agents interpret “a” and “A” differently).