Nest CSS rules inside custom properties starting with a triple-dash, ---, which represents the containing selector in the new nested rule! https://tomhodgins.com/demo/nesting/

nesting-deno.js

// deno nesting-deno.js 'a { color: red; ---\ b: { color: green }; }'
// deno nesting-deno.js --allow-read path/to/stylesheet.css
import * as parseCSS from 'https://tomhodgins.github.io/parse-css/index.js'

let file = Deno.args.slice(1)[0]
let css = file

try {
  Deno.statSync(file)
  css = new TextDecoder('utf-8').decode(
    Deno.readFileSync(file)
  )
} catch (error) {}

function expandNestedStylesheet(string, propertyName = '-') {
  const output = []

  const stringify = (list = [], joiner = '') => list
    .map(token => token.toSource())
    .join(joiner)

  function expandNestedRule(rule) {
    const output = []
    const declarations = parseCSS.parseAListOfDeclarations(
      stringify(rule.value.value)
    )

    if (declarations.length) {
      const filteredDeclarations = declarations.filter(({name}) =>
        name.startsWith(`--${propertyName}`) === false
      )

      if (filteredDeclarations.length) {
        output.push(
          parseCSS.parseARule(`
            ${stringify(rule.prelude)} {
              ${stringify(filteredDeclarations, ';')}
            }
          `)
        )
      }

      // Process nested properties
      declarations.forEach(declaration => {
        if (declaration.name.startsWith(`--${propertyName}`)) {
          output.push(
            ...expandNestedRule(
              parseCSS.parseARule(
                parseCSS.parseACommaSeparatedListOfComponentValues(
                  stringify(rule.prelude).trim()
                ).map(splitSelector =>
                  parseCSS.parseACommaSeparatedListOfComponentValues(
                    declaration.name.slice(2 + propertyName.length)
                  ).map(selector => stringify(splitSelector) + stringify(selector)).join(', ')
                ).join(', ')
                + stringify(declaration.value)
              )
            )
          )
        }
      })
    } else {
      output.push(rule)
    }

    return output
  }

  function expandNestedAtRule(rule) {
    if (
      rule.value
      && rule.value.value
    ) {
      const output = []
      const firstColon = rule.value.value.findIndex(({tokenType}) => tokenType === ':')
      const firstAt = rule.value.value.findIndex(({tokenType}) => tokenType === 'AT-KEYWORD')
      const firstBlock = rule.value.value.findIndex(({type}) => type === 'BLOCK')
      let rules = []

      if (
        [firstAt, firstColon].every(index => index < firstBlock)
      ) {
        rules = parseCSS.parseAListOfRules(
          stringify(rule.value.value)
        )
      }

      if (rules.length) {
        rules.forEach(child => {
          output.push(
            expandNestedStylesheet(child.toSource())
          )
        })

        rule.value.value = parseCSS.parseAListOfComponentValues(
          output.join('\n')
        )
      }
    }

    return rule
  }

  // Process rules in stylesheet
  parseCSS.parseAStylesheet(string).value.forEach(rule => {
    if (rule.type === 'QUALIFIED-RULE') {
      output.push(...expandNestedRule(rule))
    }
    if (rule.type === 'AT-RULE') {
      output.push(expandNestedAtRule(rule))
    }
  })

  return stringify(output, '\n')
}

console.log(
  expandNestedStylesheet(css)
)

nesting-node.js

// mkdir node_modules && cd node_modules && git clone [email protected]:tomhodgins/parse-css.git
// node nesting-node.js 'a { color: red; ---\ b: { color: green }; }'
// node nesting-node.js path/to/stylesheet.css
const parseCSS = require('parse-css/index.cjs.js')
const fs = require('fs')

let file = process.argv.slice(2)[0]
let css = file

if (fs.existsSync(file)) {
  css = fs.readFileSync(file).toString()
}

function expandNestedStylesheet(string, propertyName = '-') {
  const output = []

  const stringify = (list = [], joiner = '') => list
    .map(token => token.toSource())
    .join(joiner)

  function expandNestedRule(rule) {
    const output = []
    const declarations = parseCSS.parseAListOfDeclarations(
      stringify(rule.value.value)
    )

    if (declarations.length) {
      const filteredDeclarations = declarations.filter(({name}) =>
        name.startsWith(`--${propertyName}`) === false
      )

      if (filteredDeclarations.length) {
        output.push(
          parseCSS.parseARule(`
            ${stringify(rule.prelude)} {
              ${stringify(filteredDeclarations, ';')}
            }
          `)
        )
      }

      // Process nested properties
      declarations.forEach(declaration => {
        if (declaration.name.startsWith(`--${propertyName}`)) {
          output.push(
            ...expandNestedRule(
              parseCSS.parseARule(
                parseCSS.parseACommaSeparatedListOfComponentValues(
                  stringify(rule.prelude).trim()
                ).map(splitSelector =>
                  parseCSS.parseACommaSeparatedListOfComponentValues(
                    declaration.name.slice(2 + propertyName.length)
                  ).map(selector => stringify(splitSelector) + stringify(selector)).join(', ')
                ).join(', ')
                + stringify(declaration.value)
              )
            )
          )
        }
      })
    } else {
      output.push(rule)
    }

    return output
  }

  function expandNestedAtRule(rule) {
    if (
      rule.value
      && rule.value.value
    ) {
      const output = []
      const firstColon = rule.value.value.findIndex(({tokenType}) => tokenType === ':')
      const firstAt = rule.value.value.findIndex(({tokenType}) => tokenType === 'AT-KEYWORD')
      const firstBlock = rule.value.value.findIndex(({type}) => type === 'BLOCK')
      let rules = []

      if (
        [firstAt, firstColon].every(index => index < firstBlock)
      ) {
        rules = parseCSS.parseAListOfRules(
          stringify(rule.value.value)
        )
      }

      if (rules.length) {
        rules.forEach(child => {
          output.push(
            expandNestedStylesheet(child.toSource())
          )
        })

        rule.value.value = parseCSS.parseAListOfComponentValues(
          output.join('\n')
        )
      }
    }

    return rule
  }

  // Process rules in stylesheet
  parseCSS.parseAStylesheet(string).value.forEach(rule => {
    if (rule.type === 'QUALIFIED-RULE') {
      output.push(...expandNestedRule(rule))
    }
    if (rule.type === 'AT-RULE') {
      output.push(expandNestedAtRule(rule))
    }
  })

  return stringify(output, '\n')
}

console.log(
  expandNestedStylesheet(css)
)

test-1-input.css

a {
  level: 1;
  ---\ b: {
    level: 2;
    ---\ c: {
      level: 3;
      ---\ d: {
        level: 4;
        ---\ e: {
          level: 5;
          ---\ f: {
            level: 6;
            ---\ g: {
              level: 7;
              ---\ h: {
                level: 8;
                ---\ i: {
                  level: 9;
                  ---\ j: {
                    level: 10;
                    ---\ k: {
                      level: 11;
                      ---\ l: {
                        level: 12;
                        ---\ m: {
                          level: 13;
                          ---\ n: {
                            level: 14;
                            ---\ o: {
                              level: 15;
                              ---\ p: {
                                level: 16;
                                ---\ q: {
                                  level: 17;
                                  ---\ r: {
                                    level: 18;
                                    ---\ s: {
                                      level: 19;
                                      ---\ t: {
                                        level: 20;
                                        ---\ u: {
                                          level: 21;
                                          ---\ v: {
                                            level: 22;
                                            ---\ w: {
                                              level: 23;
                                              ---\ x: {
                                                level: 24;
                                                ---\ y: {
                                                  level: 25;
                                                  ---\ z: {
                                                    level: 26;
                                                  };
                                                };
                                              };
                                            };
                                          };
                                        };
                                      };
                                    };
                                  };
                                };
                              };
                            };
                          };
                        };
                      };
                    };
                  };
                };
              };
            };
          };
        };
      };
    };
  };
}

test-1-output.css

a { level: 1 }
a b { level: 2 }
a b c { level: 3 }
a b c d { level: 4 }
a b c d e { level: 5 }
a b c d e f { level: 6 }
a b c d e f g { level: 7 }
a b c d e f g h { level: 8 }
a b c d e f g h i { level: 9 }
a b c d e f g h i j { level: 10 }
a b c d e f g h i j k { level: 11 }
a b c d e f g h i j k l { level: 12 }
a b c d e f g h i j k l m { level: 13 }
a b c d e f g h i j k l m n { level: 14 }
a b c d e f g h i j k l m n o { level: 15 }
a b c d e f g h i j k l m n o p { level: 16 }
a b c d e f g h i j k l m n o p q { level: 17 }
a b c d e f g h i j k l m n o p q r { level: 18 }
a b c d e f g h i j k l m n o p q r s { level: 19 }
a b c d e f g h i j k l m n o p q r s t { level: 20 }
a b c d e f g h i j k l m n o p q r s t u { level: 21 }
a b c d e f g h i j k l m n o p q r s t u v { level: 22 }
a b c d e f g h i j k l m n o p q r s t u v w { level: 23 }
a b c d e f g h i j k l m n o p q r s t u v w x { level: 24 }
a b c d e f g h i j k l m n o p q r s t u v w x y { level: 25 }
a b c d e f g h i j k l m n o p q r s t u v w x y z { level: 26 }

test-2-input.css

h1, h2, h3, h4, h5, h6 {
  ---\ \+: {
    ---\ h1\,\ h2\,\ h3\,\ h4\,\ h5\,\ h6: {
      margin-top: .5em;
    };
  };
}

test-2-output.css

h1 + h1,
h1 + h2,
h1 + h3,
h1 + h4,
h1 + h5,
h1 + h6,
h2 + h1,
h2 + h2,
h2 + h3,
h2 + h4,
h2 + h5,
h2 + h6,
h3 + h1,
h3 + h2,
h3 + h3,
h3 + h4,
h3 + h5,
h3 + h6,
h4 + h1,
h4 + h2,
h4 + h3,
h4 + h4,
h4 + h5,
h4 + h6,
h5 + h1,
h5 + h2,
h5 + h3,
h5 + h4,
h5 + h5,
h5 + h6,
h6 + h1,
h6 + h2,
h6 + h3,
h6 + h4,
h6 + h5,
h6 + h6 {
  margin-top: 0.5em;
}

test-3-input.css

1 {
  ---a\,b: {
    color: cyan;
  }
}

2, 3 {
  ---c: {
    color: magenta;
  }
}

4, 5 {
  ---d\,e: {
    color: yellow;
  }
}

test-3-output.css

1a,
1b {
  color: cyan;
}
2c,
3c {
  color: magenta;
}
4d,
4e,
5d,
5e {
  color: yellow;
}

test-4-input.css

@media print {
  a {
    color: black;
    text-decoration: underline;
    ---\:\:after: {
      content: ' [link: ' attr(href) ' ]';
    };
  }
}

test-4-output.css

@media print {
  a {
    color: black;
    text-decoration: underline;
  }
  a::after {
    content: ' [link: ' attr(href) ' ]';
  }
}