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

The backslash character () is used to escape special characters in CSS, such as the percentage sign (%)$. This allows identifiers to be used that would not otherwise be valid, such as those that contain certain punctuation characters.

In the example provided, the backslash is used to escape the percentage sign in the class name .container.\31 25\25. This results in the class name being equivalent to .container[class ~= "125%"], meaning that any element with the class "125%" will have the same style rules applied to it.

This technique can be used to create dynamic class names based on various criteria. For example, the following code could be used to create a series of classes that set the width of an element based on its container size:

.container.\31 25\25 {
  width: 100%;
  max-width: 1500px;  /* max-width: (containers * 1.25) */
  min-width: 1200px;  /* min-width: (containers) */
}
.container.\37 5\25 { /* 75% */
  width: 900px;       /* width: (containers * 0.75) */
}
.container.\35 0\25 { /* 50% */
  width: 600px;       /* width: (containers * 0.50) */
}
.container.\32 5\25 { /* 25% */
  width: 300px;       /* width: (containers * 0.25) */
}

This allows the width of an element to be easily adjusted based on its container size, without having to manually create multiple classes with different percentage values.