Sorry, I was wrong. It seem that they cannot <i>start</i> with a number but you can use a number in them so long as your class start with a valid character. I&#x27;m however worried at the use of the : character.<p>The style definition might be ignored in some browsers. Some other might break when they see an invalid character and stop parsing your styles.<p>That being said, most modern browser will be able to parse the CSS without you needing to escape any characters.