diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2768ffbc4..710bbd8d7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix relative `@import` and `@plugin` paths resolving from the wrong directory when using `@tailwindcss/vite` ([#19965](https://github.com/tailwindlabs/tailwindcss/pull/19965))
 - Ensure CSS files containing `@variant` are processed by `@tailwindcss/vite` ([#19966](https://github.com/tailwindlabs/tailwindcss/pull/19966))
 - Resolve imports relative to `base` when `result.opts.from` is not provided when using `@tailwindcss/postcss` ([#19980](https://github.com/tailwindlabs/tailwindcss/pull/19980))
+- Canonicalization: preserve significant `_` whitespace in arbitrary values ([#19986](https://github.com/tailwindlabs/tailwindcss/pull/19986))
+- Canonicalization: add parentheses when removing whitespace from arbitrary values would hurt readability ([#19986](https://github.com/tailwindlabs/tailwindcss/pull/19986))
 
 ## [4.2.4] - 2026-04-21
 
diff --git a/packages/tailwindcss/src/candidate.ts b/packages/tailwindcss/src/candidate.ts
index 4e79b818e..f3ff00f52 100644
--- a/packages/tailwindcss/src/candidate.ts
+++ b/packages/tailwindcss/src/candidate.ts
@@ -3,6 +3,7 @@ import { decodeArbitraryValue } from './utils/decode-arbitrary-value'
 import { DefaultMap } from './utils/default-map'
 import { isValidArbitrary } from './utils/is-valid-arbitrary'
 import { segment } from './utils/segment'
+import type { ValueFunctionNode } from './value-parser'
 import * as ValueParser from './value-parser'
 import { walk, WalkAction } from './walk'
 
@@ -1035,26 +1036,51 @@ const printArbitraryValueCache = new DefaultMap<string, string>((input) => {
 
   let drop = new Set<ValueParser.ValueAstNode>()
 
+  let symbols = new Set([
+    // Selectors
+    '~', // Subsequent sibling combinator
+    '>', // Child combinator
+
+    // Math operators
+    '+', // or next sibling combinator
+    '-',
+    '*', // or universal selector
+    '/',
+  ])
   walk(ast, (node, ctx) => {
     let parentArray = ctx.parent === null ? ast : (ctx.parent.nodes ?? [])
 
     // Handle operators (e.g.: inside of `calc(…)`)
-    if (
-      node.kind === 'word' &&
-      // Operators
-      (node.value === '+' || node.value === '-' || node.value === '*' || node.value === '/')
-    ) {
+    if (node.kind === 'word' && symbols.has(node.value)) {
       let idx = parentArray.indexOf(node) ?? -1
 
       // This should not be possible
       if (idx === -1) return
 
+      // a + b
+      //   ^ node
+      //  ^ previous (whitespace)
       let previous = parentArray[idx - 1]
       if (previous?.kind !== 'separator' || previous.value !== ' ') return
 
+      // a + b
+      //   ^ node
+      //    ^ next (whitespace)
       let next = parentArray[idx + 1]
       if (next?.kind !== 'separator' || next.value !== ' ') return
 
+      // a + b
+      //   ^ node
+      // ^ previous (node)
+      let previousPrevious = parentArray[idx - 2]
+      if (previousPrevious && symbols.has(previousPrevious.value)) return
+
+      // a + b
+      //   ^ node
+      //     ^ next (node)
+      let nextNext = parentArray[idx + 2]
+      if (nextNext && symbols.has(nextNext.value)) return
+
       drop.add(previous)
       drop.add(next)
     }
@@ -1071,6 +1097,41 @@ const printArbitraryValueCache = new DefaultMap<string, string>((input) => {
     else if (node.kind === 'separator' && node.value.trim() === ',') {
       node.value = ','
     }
+
+    // Wrap custom functions starting with `--`, in parentheses if preceeded by
+    // a symbol. E.g.: `calc(100%---spacing(2))` → `calc(100%-(--spacing(2)))`
+    else if (node.kind === 'function' && node.value.startsWith('--')) {
+      let idx = parentArray.indexOf(node) ?? -1
+
+      // When it's the first argument, then we don't have to wrap it in `(…)`
+      //
+      // E.g.: `--spacing(…)`, no need to wrap
+      //       `calc(100%---spacing(…))`, we want to wrap
+      //
+      if (idx <= 0) return
+
+      // When it's not the first argument, we likely want to wrap it. However,
+      // when it's separated by a `,` then it's readable enough.
+      //
+      // E.g.: `min(100%,--spacing(2))` is readable, in fact
+      //       `min(100%,(--spacing(2)))` would make it worse
+      let previous = parentArray[idx - 1]
+      if (previous?.kind === 'separator' && previous.value === ',') return
+
+      // When it's part of a bigger list, aka no special symbols were used, then
+      // we don't have to wrap it either.
+      //
+      // E.g.: `shadow-[inset_0px_1px_--theme(--color-white/15%)]`, wrapping would look unnecessary:
+      //       `shadow-[inset_0px_1px_(--theme(--color-white/15%))]`
+      let previousPrevious = parentArray[idx - 2]
+      if (previousPrevious && !symbols.has(previousPrevious.value)) return
+
+      return WalkAction.ReplaceSkip({
+        kind: 'function',
+        value: '', // Unnamed, so will result in `(…)`
+        nodes: [node],
+      } satisfies ValueFunctionNode)
+    }
   })
 
   if (drop.size > 0) {
diff --git a/packages/tailwindcss/src/canonicalize-candidates.test.ts b/packages/tailwindcss/src/canonicalize-candidates.test.ts
index df029834f..6c1821baa 100644
--- a/packages/tailwindcss/src/canonicalize-candidates.test.ts
+++ b/packages/tailwindcss/src/canonicalize-candidates.test.ts
@@ -1186,6 +1186,30 @@ describe.each([['default'], ['with-variant'], ['important'], ['prefix']])('%s',
       expected,
     )
   })
+
+  test.each([
+    // Keep whitespace characters that are significant
+    ['[&:has(~_*_*:checked)]:flex', '[&:has(~_*_*:checked)]:flex'],
+    [
+      'shadow-[inset_0px_1px_--theme(--color-white/15%)]',
+      'shadow-[inset_0px_1px_--theme(--color-white/15%)]',
+    ],
+
+    // Improve readability when whitespace was used for readability
+    ['w-[calc(100%_-_calc(var(--spacing)*60))]', 'w-[calc(100%-(--spacing(60)))]'],
+    ['w-[calc(100%_-_--spacing(60))]', 'w-[calc(100%-(--spacing(60)))]'],
+
+    // No need to to wrap in `(…)` after a `,`
+    ['m-[min(100%,_--spacing(6))]', 'm-[min(100%,--spacing(6))]'],
+    ['m-[min(100%_,_--spacing(6))]', 'm-[min(100%,--spacing(6))]'],
+    ['m-[min(100%,--spacing(6))]', 'm-[min(100%,--spacing(6))]'],
+  ])(testName, async (candidate, expected) => {
+    let input = css`
+      @import 'tailwindcss';
+    `
+
+    await expectCanonicalization(input, candidate, expected)
+  })
 })
 
 describe('theme to var', () => {