ID’s Cannot Start With a Number
by Chris Coyier
Tempted to use something like "1800number_box" for a ID? Don't. Because it won't work. Class identifiers are allowed to start with a number, but ID identifiers are not.
That is all.
If you have already existing code using it that way that you absolutely can't change, you can access it with:
/* does work */
/* doesn't work */
*May or may not contain any actual "CSS" or "Tricks".