#css# id & class 不能以數字開頭

[id='1800number_box'] {
 /* does work */
}

#1800number_box {
  /* doesn't work */
}

/* Or, you can "escape" the number, which looks funky but works: */
#\31 800number_box {
  /* does work */
}

[class="10"] {color: red;}

html5可以支持以數字開頭的id／class，但是css不支持：

In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-zA-Z0-9] and ISO 10646 characters U+00A0 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, two hyphens, or a hyphen followed by a digit. Identifiers can also contain escaped characters and any ISO 10646 character as a numeric code (see next item). For instance, the identifier “B&W?” may be written as “B\&W\?” or “B\26 W\3F”.

W3C Specification