diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 000000000..25358efc6
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,40 @@
+# Repository Guidelines
+
+## Project Structure & Module Organization
+- `packages/*` — TypeScript packages. Core lives in `packages/tailwindcss`; adapters/tools are under `@tailwindcss-*`.
+- `crates/*` — Rust crates (e.g. `crates/oxide`, `crates/node`). Node bindings are published from here.
+- `integrations/` — Vitest-based integration tests for CLIs/build tools.
+- `scripts/` — Release and maintenance utilities (e.g. `version-packages.mjs`).
+- `dist/` — Packed artifacts after builds; useful for local installs.
+- `playgrounds/*` — Sample apps to manually verify changes.
+
+## Build, Test, and Development Commands
+- `pnpm install` — Install JS deps. Ensure Rust toolchain via `rustup default stable` and add `wasm32-wasip1-threads`.
+- `pnpm dev` — Turbo watch builds; filter example: `pnpm dev --filter=tailwindcss`.
+- `pnpm build` — Build all packages (excludes playgrounds). Creates tarballs in `dist/`.
+- `pnpm test` — Run Rust `cargo test` and Vitest across the workspace.
+- `pnpm test:integrations` — Run integration tests (run `pnpm build` first).
+- `pnpm test:ui` — Run Playwright UI tests for browser-related behavior.
+- `pnpm lint` / `pnpm format` — Check/fix formatting and package-level lint scripts.
+
+## Coding Style & Naming Conventions
+- Prettier is authoritative: `semi: false`, `singleQuote: true`, `printWidth: 100`.
+- Indentation: 2 spaces. TypeScript files prefer `.ts`; compatibility shims may use `.cts`.
+- Naming: kebab-case file names, camelCase functions/vars, PascalCase types/classes. Rust modules use `snake_case`.
+- Run `pnpm lint` locally before opening a PR.
+
+## Testing Guidelines
+- Unit tests: Vitest; name files `*.test.ts` near the code they cover or in `src/**/__tests__`.
+- Integration: tests live in `integrations/**`; run `pnpm build && pnpm test:integrations`.
+- UI: Playwright tests via `pnpm test:ui`.
+- Rust: crate tests run with `cargo test` (also covered by `pnpm test`). Include tests with any behavior change.
+
+## Commit & Pull Request Guidelines
+- Commit messages: clear, present-tense summaries; reference issues like `#123` when relevant.
+- New features: open a discussion before investing significant work.
+- PRs must include: problem summary, rationale, and a test plan (exact commands; screenshots if UI-related). Ensure all tests pass; use `[ci-all]` in the description to run all platform suites.
+- See `.github/CONTRIBUTING.md` and `.github/PULL_REQUEST_TEMPLATE.md` for details.
+
+## Security & Configuration Tips
+- Do not commit secrets or machine-specific paths. Use `pnpm@9` (workspace-managed) and Rust stable. Turbo caches builds; when in doubt, rebuild with `pnpm build`.
+
diff --git a/packages/tailwindcss/src/ast.ts b/packages/tailwindcss/src/ast.ts
index a473b9193..006d2ef30 100644
--- a/packages/tailwindcss/src/ast.ts
+++ b/packages/tailwindcss/src/ast.ts
@@ -4,6 +4,7 @@ import type { DesignSystem } from './design-system'
 import type { Source, SourceLocation } from './source-maps/source'
 import { Theme, ThemeOptions } from './theme'
 import { DefaultMap } from './utils/default-map'
+import { segment } from './utils/segment'
 import { extractUsedVariables } from './utils/variables'
 import * as ValueParser from './value-parser'
 
@@ -662,7 +663,11 @@ export function optimizeAst(
           ...declaration,
           value: ValueParser.toCss(ast),
         }
-        let colorMixQuery = rule('@supports (color: color-mix(in lab, red, red))', [declaration])
+        let colorMixQuery = rule('@supports (color: color-mix(in lab, red, red))', [
+          // Keep the original declaration behind a `@supports` rule and ensure
+          // it remains scoped to the current selector by wrapping it in `&`.
+          rule('&', [declaration]),
+        ])
         colorMixQuery.src = declaration.src
         parent.splice(idx, 1, fallback, colorMixQuery)
       }
@@ -729,9 +734,269 @@ export function optimizeAst(
     }
   }
 
+  // Flatten nested style-rule selectors globally so stringify produces
+  // standards-compliant CSS without nesting. This must run after we insert
+  // polyfills that depend on the original parent/child arrays created during
+  // transformation, otherwise we would be mutating stale arrays.
+  newAst = flattenAst(newAst)
+
   return newAst
 }
 
+export function flattenAst(ast: AstNode[]): AstNode[] {
+  function splitSelectors(sel: string): string[] {
+    return segment(sel, ',')
+      .map((s) => s.trim())
+      .filter(Boolean)
+  }
+
+  function isRelative(child: string): boolean {
+    let s = child.trim()
+    return s.startsWith('>') || s.startsWith('+') || s.startsWith('~')
+  }
+
+  function normalizeSpaces(s: string): string {
+    return s.replace(/\s+/g, ' ').trim()
+  }
+
+  function normalizePseudoElements(s: string): string {
+    // Move known pseudo-elements to the end of the last compound selector
+    let parts = s.split(/(\s*[>+~]\s*)/)
+    let last = parts[parts.length - 1]
+    let pseudos: string[] = []
+    last = last.replace(
+      /:{1,2}(before|after|first-letter|first-line|marker|backdrop|details-content)\b/g,
+      (m) => {
+        pseudos.push(m)
+        return ''
+      },
+    )
+    if (pseudos.length > 0) parts[parts.length - 1] = `${last}${pseudos.join('')}`
+    return parts.join('')
+  }
+
+  function combine(parentList: string[] | null, childList: string[]): string[] {
+    if (!parentList || parentList.length === 0) return childList.map(normalizeSpaces)
+
+    let out: string[] = []
+    for (let p of parentList) {
+      for (let c of childList) {
+        let combined: string
+        if (c.includes('&')) {
+          combined = normalizeSpaces(c.replaceAll('&', p))
+        } else if (isRelative(c)) {
+          combined = normalizeSpaces(`${p} ${c}`)
+        } else {
+          combined = normalizeSpaces(`${p} ${c}`)
+        }
+        out.push(normalizePseudoElements(combined))
+      }
+    }
+    return out
+  }
+
+  function flatten(nodes: AstNode[], parentSelectors: string[] | null): AstNode[] {
+    let result: AstNode[] = []
+
+    for (let node of nodes) {
+      if (node.kind === 'rule') {
+        let selfList = splitSelectors(node.selector)
+        let current = parentSelectors
+          ? combine(parentSelectors, selfList)
+          : selfList.map(normalizeSpaces)
+
+        let bufferDecls: AstNode[] = []
+        let flush = () => {
+          if (bufferDecls.length === 0) return
+          for (let sel of current) {
+            result.push({
+              kind: 'rule',
+              selector: normalizePseudoElements(sel),
+              nodes: bufferDecls.slice(),
+            })
+          }
+          bufferDecls = []
+        }
+
+        for (let child of node.nodes) {
+          if (child.kind === 'declaration' || child.kind === 'comment') {
+            bufferDecls.push(child)
+            continue
+          }
+
+          // We hit a nested block; flush accumulated decls first to preserve order
+          flush()
+
+          if (child.kind === 'rule') {
+            result.push(...flatten([child], current))
+          } else if (child.kind === 'at-rule') {
+            // Ensure declarations inside nested at-rules remain scoped to the
+            // current selector by first wrapping them in a synthetic rule and
+            // then flattening. This prevents emitting invalid CSS like
+            // `@media{ color:red }` without a selector.
+            let inner = flatten([styleRule('&', child.nodes)], current)
+            if (
+              inner.length > 0 ||
+              child.name === '@layer' ||
+              child.name === '@charset' ||
+              child.name === '@custom-media' ||
+              child.name === '@namespace' ||
+              child.name === '@import'
+            ) {
+              result.push({ ...child, nodes: inner })
+            }
+          }
+        }
+
+        // Flush remaining decls after processing nested children
+        flush()
+      } else if (node.kind === 'at-rule') {
+        let inner = flatten(node.nodes, parentSelectors)
+        if (
+          inner.length > 0 ||
+          node.name === '@layer' ||
+          node.name === '@charset' ||
+          node.name === '@custom-media' ||
+          node.name === '@namespace' ||
+          node.name === '@import'
+        ) {
+          result.push({ ...node, nodes: inner })
+        }
+      } else {
+        result.push(node)
+      }
+    }
+
+    return result
+  }
+
+  function dedupeDecls(nodes: AstNode[]): AstNode[] {
+    let seen = new Set<string>()
+    let out: AstNode[] = []
+    for (let n of nodes) {
+      if (n.kind !== 'declaration') {
+        out.push(n)
+        continue
+      }
+      let key = `${n.property}\u0001${n.value ?? ''}\u0001${n.important ? '!' : ''}`
+      if (seen.has(key)) continue
+      seen.add(key)
+      out.push(n)
+    }
+    return out
+  }
+
+  function mergeRules(nodes: AstNode[]): AstNode[] {
+    let result: AstNode[] = []
+    // Only merge rules within contiguous segments so we don't reorder
+    // content across at-rule boundaries (e.g. keep rules before/after
+    // an intervening @supports or @media block separate).
+    let bySelector = new Map<string, StyleRule>()
+
+    for (let node of nodes) {
+      if (node.kind === 'rule') {
+        let existing = bySelector.get(node.selector)
+        if (existing) {
+          existing.nodes = dedupeDecls(existing.nodes.concat(node.nodes))
+          continue
+        }
+        let copy: StyleRule = { ...node, nodes: dedupeDecls(node.nodes.slice()) }
+        bySelector.set(copy.selector, copy)
+        result.push(copy)
+      } else if (node.kind === 'at-rule') {
+        let mergedChildren = mergeRules(node.nodes)
+        result.push({ ...node, nodes: mergedChildren })
+        // Reset the merge cache across at-rule boundaries to preserve
+        // ordering of selectors before/after the block.
+        bySelector = new Map()
+      } else {
+        result.push(node)
+      }
+    }
+
+    return result
+  }
+
+  function canRecombine(nodes: AstNode[]): boolean {
+    for (let n of nodes) {
+      if (n.kind !== 'declaration' && n.kind !== 'comment') return false
+    }
+    return true
+  }
+
+  function fingerprint(nodes: AstNode[]): string | null {
+    if (!canRecombine(nodes)) return null
+    let parts: string[] = []
+    for (let n of nodes) {
+      if (n.kind === 'declaration') {
+        parts.push(`d\u0001${n.property}\u0001${n.value ?? ''}\u0001${n.important ? '!' : ''}`)
+      } else if (n.kind === 'comment') {
+        parts.push(`c\u0001${n.value}`)
+      }
+    }
+    return parts.join('\u0002')
+  }
+
+  function recombineAdjacentRules(nodes: AstNode[]): AstNode[] {
+    let out: AstNode[] = []
+    for (let node of nodes) {
+      if (node.kind === 'at-rule') {
+        let mergedChildren = recombineAdjacentRules(node.nodes)
+
+        // Merge adjacent conditional at-rules with identical name/params
+        // to reduce duplication like:
+        //   @media foo { … }
+        //   @media foo { … }
+        // becomes:
+        //   @media foo { … … }
+        function canMergeAtRule(name: string) {
+          return name === '@media' || name === '@supports' || name === '@container'
+        }
+
+        if (
+          out.length > 0 &&
+          out[out.length - 1].kind === 'at-rule' &&
+          (out[out.length - 1] as AtRule).name === node.name &&
+          (out[out.length - 1] as AtRule).params === node.params &&
+          canMergeAtRule(node.name)
+        ) {
+          let prev = out[out.length - 1] as AtRule
+          prev.nodes = recombineAdjacentRules(prev.nodes.concat(mergedChildren))
+          continue
+        }
+
+        out.push({ ...node, nodes: mergedChildren })
+        continue
+      }
+
+      if (node.kind !== 'rule') {
+        out.push(node)
+        continue
+      }
+
+      let fp = fingerprint(node.nodes)
+      if (
+        fp &&
+        out.length > 0 &&
+        out[out.length - 1].kind === 'rule' &&
+        fingerprint((out[out.length - 1] as StyleRule).nodes) === fp
+      ) {
+        let prev = out[out.length - 1] as StyleRule
+        prev.selector = `${prev.selector}, ${node.selector}`
+        continue
+      }
+
+      out.push(node)
+    }
+    return out
+  }
+
+  let flattened = flatten(ast, null)
+  let merged = mergeRules(flattened)
+  let recombined = recombineAdjacentRules(merged)
+  return recombined
+}
+
 export function toCss(ast: AstNode[], track?: boolean) {
   let pos = 0
 
diff --git a/packages/tailwindcss/src/index.test.ts b/packages/tailwindcss/src/index.test.ts
index cfd0d1db8..abc8ea731 100644
--- a/packages/tailwindcss/src/index.test.ts
+++ b/packages/tailwindcss/src/index.test.ts
@@ -2,7 +2,9 @@ import fs from 'node:fs'
 import path from 'node:path'
 import { describe, expect, it, test } from 'vitest'
 import { compile, Features, Polyfills } from '.'
+import { flattenAst, toCss as toCssAst } from './ast'
 import type { PluginAPI } from './compat/plugin-api'
+import * as CSS from './css-parser'
 import plugin from './plugin'
 import { compileCss, optimizeCss, run } from './test-utils/run'
 
@@ -5966,3 +5968,191 @@ describe('feature detection', () => {
     expect(compiler.features & Features.Utilities).toBeFalsy()
   })
 })
+
+describe('CSS flattening', () => {
+  it('pseudo-element last: &::before:hover -> :hover::before', () => {
+    let ast = CSS.parse(css`
+      .btn {
+        &::before:hover {
+          content: '';
+        }
+      }
+    `)
+    let flat = flattenAst(ast)
+    expect(toCssAst(flat)).toMatchInlineSnapshot(`
+      ".btn:hover::before {\n  content: '';\n}\n"
+    `)
+  })
+
+  it('selector-list cross product (parent × child)', () => {
+    let ast = CSS.parse(css`
+      .a,
+      .b {
+        &:hover,
+        &:focus {
+          opacity: 0.5;
+        }
+      }
+    `)
+    let flat = flattenAst(ast)
+    expect(toCssAst(flat)).toMatchInlineSnapshot(`
+      ".a:hover, .a:focus, .b:hover, .b:focus {
+        opacity: 0.5;
+      }
+      "
+    `)
+  })
+
+  it('descendant join when & is omitted', () => {
+    let ast = CSS.parse(css`
+      .a,
+      .b {
+        .child,
+        .c2 {
+          color: blue;
+        }
+      }
+    `)
+    let flat = flattenAst(ast)
+    expect(toCssAst(flat)).toMatchInlineSnapshot(`
+      ".a .child, .a .c2, .b .child, .b .c2 {
+        color: blue;
+      }
+      "
+    `)
+  })
+
+  it('relative combinators (>, +, ~)', () => {
+    let ast = CSS.parse(css`
+      .a {
+        > .b,
+        + .c,
+        ~ .d {
+          color: red;
+        }
+      }
+    `)
+    let flat = flattenAst(ast)
+    expect(toCssAst(flat)).toMatchInlineSnapshot(`
+      ".a > .b, .a + .c, .a ~ .d {
+        color: red;
+      }
+      "
+    `)
+  })
+
+  it('multiple &: & + & over lists', () => {
+    let ast = CSS.parse(css`
+      .a,
+      .b {
+        & + & {
+          color: green;
+        }
+      }
+    `)
+    let flat = flattenAst(ast)
+    expect(toCssAst(flat)).toMatchInlineSnapshot(`
+      ".a + .a, .b + .b {
+        color: green;
+      }
+      "
+    `)
+  })
+
+  it('preserves :is/:where groupings', () => {
+    let ast = CSS.parse(css`
+      .a,
+      .b {
+        &:is(:hover, :focus) {
+          opacity: 0.5;
+        }
+        :where(.x, .y) {
+          color: red;
+        }
+      }
+    `)
+    let flat = flattenAst(ast)
+    expect(toCssAst(flat)).toMatchInlineSnapshot(`
+      ".a:is(:hover, :focus), .b:is(:hover, :focus) {
+        opacity: 0.5;
+      }
+      .a :where(.x, .y), .b :where(.x, .y) {
+        color: red;
+      }
+      "
+    `)
+  })
+
+  it('combines duplicate rules and de-dupes declarations', () => {
+    let ast = CSS.parse(css`
+      .a {
+        & {
+          color: red;
+          color: red;
+        }
+        & {
+          background: blue;
+        }
+      }
+    `)
+    let flat = flattenAst(ast)
+    expect(toCssAst(flat)).toMatchInlineSnapshot(`
+      ".a {\n  color: red;\n  background: blue;\n}\n"
+    `)
+  })
+
+  it('nested @media wrappers preserved', () => {
+    let ast = CSS.parse(css`
+      .a,
+      .b {
+        @media (min-width: 768px) {
+          &:hover {
+            color: red;
+          }
+        }
+      }
+    `)
+    let flat = flattenAst(ast)
+    expect(toCssAst(flat)).toMatchInlineSnapshot(`
+      "@media (min-width: 768px) {
+        .a:hover, .b:hover {
+          color: red;
+        }
+      }
+      "
+    `)
+  })
+
+  it('recombines blocks nested in multiple @media rules', () => {
+    let ast = CSS.parse(css`
+      .x {
+        @media foo {
+          @media bar {
+            color: red;
+          }
+        }
+      }
+      .y {
+        @media foo {
+          @media bar {
+            color: blue;
+          }
+        }
+      }
+    `)
+    let flat = flattenAst(ast)
+    expect(toCssAst(flat)).toMatchInlineSnapshot(`
+      "@media foo {
+        @media bar {
+          .x {
+            color: red;
+          }
+          .y {
+            color: blue;
+          }
+        }
+      }
+      "
+    `)
+  })
+})
diff --git a/packages/tailwindcss/src/index.ts b/packages/tailwindcss/src/index.ts
index ca2a0b0df..28857baaf 100644
--- a/packages/tailwindcss/src/index.ts
+++ b/packages/tailwindcss/src/index.ts
@@ -399,7 +399,10 @@ async function parseCss(
               let nodes: AstNode[] = []
 
               if (styleRuleSelectors.length > 0) {
-                nodes.push(styleRule(styleRuleSelectors.join(', '), r.nodes))
+                // Preserve specificity across a selector list by wrapping in :is(…)
+                // so later flattening/combination doesn’t turn this into two
+                // separate rules with potentially differing specificity.
+                nodes.push(styleRule(`:is(${styleRuleSelectors.join(', ')})`, r.nodes))
               }
 
               for (let selector of atRuleParams) {
diff --git a/packages/tailwindcss/src/utilities.ts b/packages/tailwindcss/src/utilities.ts
index efe7b3dc6..b515190ad 100644
--- a/packages/tailwindcss/src/utilities.ts
+++ b/packages/tailwindcss/src/utilities.ts
@@ -6361,7 +6361,9 @@ function alphaReplacedShadowProperties(
   if (requiresFallback) {
     return [
       decl(property, applyPrefix(replaceShadowColors(value, varInjector))),
-      rule('@supports (color: lab(from red l a b))', [decl(property, applyPrefix(replacedValue))]),
+      rule('@supports (color: lab(from red l a b))', [
+        rule('&', [decl(property, applyPrefix(replacedValue))]),
+      ]),
     ]
   } else {
     return [decl(property, applyPrefix(replacedValue))]
@@ -6412,7 +6414,9 @@ function alphaReplacedDropShadowProperties(
             .map((value) => `drop-shadow(${replaceShadowColors(value, varInjector)})`)
             .join(' '),
       ),
-      rule('@supports (color: lab(from red l a b))', [decl(property, prefix + replacedValue)]),
+      rule('@supports (color: lab(from red l a b))', [
+        rule('&', [decl(property, prefix + replacedValue)]),
+      ]),
     ]
   } else {
     return [decl(property, prefix + replacedValue)]