What's the Purpose of the Backslash Character in CSS's ".container.\31 25\25"?

In the provided CSS code, the backslash character () serves a specific purpose related to identifier escaping. According to the CSS specification:

• Escaping Special Characters: A backslash can cancel the meaning of special CSS characters. In this case, it prevents the percentage sign (%) from being recognized as the start of a CSS calculation.
• Numeric Code Point References: When followed by six hexadecimal digits, a backslash can represent a specific Unicode character. This allows authors to refer to characters that are not easily entered in the document.

In the example:

.container.\31 25\25 is equivalent to .container[class ~= "125%"].

• 31 is the hexadecimal representation of the number 1.
• 25 is the hexadecimal representation of the number 17.
• \25 is the percent sign, escaped to remove its special meaning.

Therefore, the backslash escapes the percentage sign, allowing it to be treated as part of the identifier rather than a calculation. This technique ensures that the "125%" value is interpreted as a class name rather than a calculation.

Without the backslash, the identifier would start with a number and be invalid as per CSS syntax:

• .container.125% is invalid because identifiers cannot start with a number.

By using the backslash, the percentage values can be used as class names, enabling flexible styling based on container sizes.