Is there a reason why CSS doesn't support ids and classes, starting from numbers?

Question

I noticed that css properties aren't applied if id or class starts from number. For example, none of following will work:

.1ww{
    /* some properties here */
}

and

.1{
    /* some properties here */
}

and

#1{
    /* some properties here */
}

and

#1ww{
    /* some properties here */
}

Why doesn't CSS support ids or class names, starting from numbers? (Javascript, for example, works with numeric ids and classes)

Answer 1

IDs and classes must be valid identifiers.

Identifiers are listed like so in the specification:

ident       -?{nmstart}{nmchar}*

So an optional - sign, followed by nmstart, followed by any number of nmchars.

The one we're interested in is nmstart, but I'll also list nmchar:

nmstart     [_a-z]|{nonascii}|{escape}
nmchar      [_a-z0-9-]|{nonascii}|{escape}

And just for completeness:

nonascii    [240-377]
escape      {unicode}|\[^
f0-9a-f]
unicode     \{h}{1,6}(
|[ 
f])?

Okay, so with all that out of the way, notice how nmstart does not include 0-9 or -. This means that IDs and classes cannot start with a number according to the CSS specification. In fact, they can't even start with -1. Or two hyphens.

This is why they are not recognised in your code.