You can make a horizontal separator line in CSS by simply using the HTML `
HTML:
```
CSS:
```
.separator {
border: none;
border-top: 1px solid gray;
width: 50%;
margin: 10px auto;
}
```
In this example:
- `border: none;` resets the styles for border.
- `border-top: 1px solid gray;` makes a 1px solid gray line.
- `width: 50%;` means that the line should take up half the width of its container.
- `margin: 10px auto;` centers the line horizontally and gives it some vertical spacing.
You always can edit this CSS rule to customize the line to fit your design.