为什么不直接将CSS文件保存为UTF-8格式呢?

nav a:hover:after {
    content: "↓";
}

如果这还不够好，你想保持全ascii:

nav a:hover:after {
    content: "\2193";
}

Unicode字符在字符串中的一般格式是\000000到\FFFFFF—一个反斜杠后跟六个十六进制数字。当Unicode字符是字符串中的最后一个字符或在Unicode字符后添加空格时，可以省略前导0数字。有关详细信息，请参阅下面的规范。

CSS2规范的相关部分:

Third, backslash escapes allow authors to refer to characters they cannot easily put in a document. In this case, the backslash is followed by at most six hexadecimal digits (0..9A..F), which stand for the ISO 10646 ([ISO10646]) character with that number, which must not be zero. (It is undefined in CSS 2.1 what happens if a style sheet does contain a character with Unicode codepoint zero.) If a character in the range [0-9a-fA-F] follows the hexadecimal number, the end of the number needs to be made clear. There are two ways to do that: with a space (or other white space character): "\26 B" ("&B"). In this case, user agents should treat a "CR/LF" pair (U+000D/U+000A) as a single white space character. by providing exactly 6 hexadecimal digits: "\000026B" ("&B") In fact, these two methods may be combined. Only one white space character is ignored after a hexadecimal escape. Note that this means that a "real" space after the escape sequence must be doubled. If the number is outside the range allowed by Unicode (e.g., "\110000" is above the maximum 10FFFF allowed in current Unicode), the UA may replace the escape with the "replacement character" (U+FFFD). If the character is to be displayed, the UA should show a visible symbol, such as a "missing character" glyph (cf. 15.2, point 5). Note: Backslash escapes are always considered to be part of an identifier or a string (i.e., "\7B" is not punctuation, even though "{" is, and "\32" is allowed at the start of a class name, even though "2" is not). The identifier "te\st" is exactly the same identifier as "test".

综合列表:Unicode字符“向下箭头”(U+2193)。