Support CSS nesting

package.json

@@ -79,6 +79,7 @@
     "jsdoc": "^4.0.3",
     "npm-check-updates": "^17.1.0",
     "npm-run-all2": "^6.2.2",
+    "postcss-selector-parser": "^6.1.2",
     "prettier": "^3.3.3",
     "rimraf": "^6.0.1",
     "sass": "1.77.8",
@@ -88,12 +89,14 @@
     "ws": "^8.18.0"
   },
   "dependencies": {
+    "@csstools/postcss-is-pseudo-class": "^5.0.0",
     "cssesc": "^3.0.0",
     "js-yaml": "^4.1.0",
     "lodash.kebabcase": "^4.1.1",
     "markdown-it": "^14.1.0",
     "markdown-it-front-matter": "^0.2.4",
-    "postcss": "^8.4.41"
+    "postcss": "^8.4.41",
+    "postcss-nesting": "^13.0.0"
   },
   "publishConfig": {
     "access": "public"

src/marpit.js

@@ -23,6 +23,7 @@ const defaultOptions = {
   anchor: true,
   container: marpitContainer,
   cssContainerQuery: false,
+  cssNesting: true,
   headingDivider: false,
   lang: undefined,
   looseYAML: false,
@@ -73,6 +74,9 @@ class Marpit {
    *     to enable CSS container query (`@container`). By setting the string or
    *     string array, you can specify the container name(s) for the CSS
    *     container.
+   * @param {boolean} [opts.cssNesting=false] Enable CSS nesting support. If
+   *     enabled, Marpit will try to make flatten the CSS with nested rules
+   *     before rendering, to make it compatible with Marpit preprocessing.
    * @param {false|number|number[]} [opts.headingDivider=false] Start a new
    *     slide page at before of headings. it would apply to headings whose
    *     larger than or equal to the specified level if a number is given, or
@@ -128,7 +132,9 @@ class Marpit {
     /**
      * @type {ThemeSet}
      */
-    this.themeSet = new ThemeSet()
+    this.themeSet = new ThemeSet({
+      cssNesting: this.options.cssNesting,
+    })
 
     this.applyMarkdownItPlugins(
       (() => {
@@ -267,9 +273,9 @@ class Marpit {
         ...wrapArray(this.options.container),
         ...wrapArray(this.options.slideContainer),
       ],
+      containerQuery: this.options.cssContainerQuery,
       inlineSVG: this.inlineSVGOptions,
       printable: this.options.printable,
-      containerQuery: this.options.cssContainerQuery,
     }
   }
 

src/postcss/nesting.js

@@ -0,0 +1,44 @@
+/** @module */
+import postcssPlugin from '../helpers/postcss_plugin'
+import postcssNesting from 'postcss-nesting'
+import postcssIsPseudoClass from '@csstools/postcss-is-pseudo-class'
+
+const { Rule: applyPostCSSNesting } = postcssNesting()
+
+const matcher = /:is\((?:section|:root)\b/
+
+export const nesting = postcssPlugin(
+  'marpit-postcss-nesting',
+  () => (root, helpers) => {
+    const rules = []
+
+    // Note: Use walk instead of walkRules to include nested rules
+    root.walk((node) => {
+      if (node.type !== 'rule') return
+
+      rules.push(node)
+      node.__marpitNestingOriginalSelector = node.selector
+    })
+
+    // Apply postcss-nesting
+    root.walkRules((rule) => applyPostCSSNesting(rule, helpers))
+
+    const { Rule: applyPostCSSIsPseudoClass } = postcssIsPseudoClass({
+      onComplexSelector: 'warning',
+    }).prepare()
+
+    for (const rule of rules) {
+      if (
+        rule.__marpitNestingOriginalSelector !== rule.selector &&
+        matcher.test(rule.selector)
+      ) {
+        // Apply postcss-is-pseudo-class only to transformed rules that is
+        // including `:is() selector starting from `section` element or `:root`
+        // pseudo-class
+        applyPostCSSIsPseudoClass(rule, helpers)
+      }
+    }
+  },
+)
+
+export default nesting

src/theme.js

@@ -1,6 +1,7 @@
 import postcss from 'postcss'
 import postcssImportParse from './postcss/import/parse'
 import postcssMeta from './postcss/meta'
+import postcssNesting from './postcss/nesting'
 import { pseudoClass } from './postcss/root/increasing_specificity'
 import postcssRootReplace from './postcss/root/replace'
 import postcssSectionSize from './postcss/section_size'
@@ -13,6 +14,7 @@ const absoluteUnits = {
   pc: (v) => v * 16,
   pt: (v) => (v * 4) / 3,
   px: (v) => v,
+  // q: (v) => (v * 24) / 25.4,
 }
 
 const convertToPixel = (value) => {
@@ -95,16 +97,20 @@ class Theme {
    *     `@theme` meta comment.
    * @param {Object} [opts]
    * @param {Object} [opts.metaType] An object for defined types for metadata.
+   * @param {Object} [opts.cssNesting] Enable support for CSS nesting.
    */
   static fromCSS(cssString, opts = {}) {
     const metaType = { ...(opts.metaType || {}), ...reservedMetaType }
 
-    const { css, result } = postcss([
-      postcssMeta({ metaType }),
-      postcssRootReplace({ pseudoClass }),
-      postcssSectionSize({ preferedPseudoClass: pseudoClass }),
-      postcssImportParse,
-    ]).process(cssString)
+    const { css, result } = postcss(
+      [
+        !!opts.cssNesting && postcssNesting,
+        postcssMeta({ metaType }),
+        postcssRootReplace({ pseudoClass }),
+        postcssSectionSize({ preferedPseudoClass: pseudoClass }),
+        postcssImportParse,
+      ].filter((p) => p),
+    ).process(cssString)
 
     if (!opts[skipThemeValidationSymbol] && !result.marpitMeta.theme)
       throw new Error('Marpit theme CSS requires @theme meta.')