css - CSS 不支持从数字开始的 ids 和 classes 是有原因的吗？

Question

我注意到如果 id 或 class 从数字开始，则不会应用 css 属性。例如，以下均无效:

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

和

.1{
    /* some properties here */
}

和

#1{
    /* some properties here */
}

和

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

为什么 CSS 不支持从数字开始的 id 或类名？ (例如，Javascript 使用数字 ID 和类)

Answer 1

事实上，规范指出以数字开头的类将被解释为维度 [ 1 ]:

CSS2 parses a number immediately followed by an identifier as a DIMENSION token (i.e., an unknown unit), CSS1 parsed it as a number and an identifier. That means that in CSS1, the declaration 'font: 10pt/1.2serif' was correct, as was 'font: 10pt/12pt serif'; in CSS2, a space is required before "serif". (Some UAs accepted the first example, but not the second.)

并且由于我们不知道将来会引入哪些维度，所以CSS中不存在的维度被解析为未知维度:

In CSS1, a class name could start with a digit (".55ft"), unless it was a dimension (".55in"). In CSS2, such classes are parsed as unknown dimensions (to allow for future additions of new units). To make ".55ft" a valid class, CSS2 requires the first digit to be escaped (".\35 5ft")

您可以通过转义第一个数字来使用以数字开头的类，这根据 W3C CSS 验证器是有效的。查看 plunkr .

[ 1 ] 附录 G. CSS 2.1 语法