CSS - Source Map

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1 - About

For each CSS file it produces, a CSS preprocessor generates:

  • a source map file (.map)
  • and the compiled CSS.

The source map file is a JSON file that defines a mapping between:

  • each (compiled|generated) CSS declaration
  • and the corresponding line of the source file.

It allow then to map the (compiled|generated) CSS back to the original pre-processor source file.

Each CSS file contains an annotation that specifies the (URL|location) of its source map file, embedded in a special comment on the last line of the file:

/*# sourceMappingURL=<url> */

3 - Example

For instance, Sass generates:

  • from the following Sass source file (styles.scss)
$textSize: 26px;
$fontColor: red;
$bgColor: whitesmoke;
h2 {
    font-size: $textSize;
    color: $fontColor;
    background: $bgColor;
  • a CSS file (styles.css) that contains the sourceMappingURL annotation
h2 {
  font-size: 26px;
  color: red;
  background-color: whitesmoke;
/*# sourceMappingURL=styles.css.map */
  • and the source map file (styles.css.map)
  "version": "3",
  "sources": ["sass/styles.scss"],
  "file": "styles.css"

where the mappings values are encoded in VLQ (Variable Length Quantity)