Glad that this CSS selector escape stuff is not just confusing me. ;-)

http://mathiasbynens.be/notes/css-escapes

Digits are not allowed in selectors as the first character. You have to escape them with their unicode code point.

"Leading digits
If the first character of an identifier is numeric, you’ll need to escape it based on its Unicode code point. For example, the code point for the character 1 is U+0031, so you would escape it as \000031 or \31 .
Basically, to escape any numeric character, just prefix it with \3 and append a space character ( ). Yay Unicode!"

This is why I want an example in the spec, since numbers are the most common way to put an identifier on cues - maybe we should even mention that this is a unicode codepoint?