What does the .container.\31 25\25 mean in CSS?

In CSS, identifiers can contain special characters, such as the backslash (). The backslash character serves different purposes in CSS, depending on the context.

Escape Characters

Within a string, a backslash followed by a newline is ignored. Outside a string, a backslash followed by a newline represents itself.

Escaping Special Characters

A backslash can be used to escape special CSS characters, making them lose their special meaning. For example, a double quote (") within a string can be escaped as \".

Unicode Character References

A backslash followed by up to six hexadecimal digits (0-9a-fA-F) represents the ISO 10646 character with that number. White space or an additional hexadecimal digit is used to terminate the reference.

In the given code, the backslash () is being used to represent a percentage value in the class name of the .container element:

.container.\31 25\25 { /* 125% */
  width: 100%;
}

The special characters \31, 25, and \25 represent the hexadecimal numbers for the characters "1", "%", and "%", respectively. Therefore, this code is equivalent to:

.container[class ~= "125%"] { /* 125% */
  width: 100%;
}

This class selector matches any element with a class containing "125%". The calculation for the width of the element is as follows:

width = (containers * 125%)