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CSS escape

如果使用 querySelector 的话,传入的 selector 如果为数字的 class 或者 id 时会报错。但是使用 getElementById 以及 getElementByClass 则不会。
这个是因为跟 CSS 的转义有关。
It is valid, but requires some special handling. From here:
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 00031 or 1 .
Basically, to escape any numeric character, just prefix it with and append a space character ( ). Yay Unicode!
So your code would end up as (CSS first, JS second):
#\31  {
    background: hotpink;

document.querySelector('#\\31 ');