fix: exclude iframes from focus-visible auto outline in Preflight (tailwindlabs#20292)

## Description

Firefox's UA stylesheet already sets `iframe:focus-visible {
outline-style: none; }`, so Preflight's `:-moz-focusring { outline:
auto; }` rule overrides that and applies an unwanted auto outline to
focused iframes.

Adding `:where(:not(iframe))` to the selector preserves the improved
focus ring behavior for all other elements while respecting Firefox's
native iframe focus styling.

Fixes tailwindlabs#19795

Edit by @RobinMalfait 

## Test plan

Before:
<img width="1054" height="383"
alt="file-f833142116d36e1e620290b97455f001"
src="https://github.com/user-attachments/assets/81aa170b-2cef-4964-934d-61f258335e1a"
/>


After:
<img width="1056" height="310"
alt="file-400d9214fedf5b43693e10580a4869de"
src="https://github.com/user-attachments/assets/8f8bb81d-6070-425d-8820-327bf9e88e79"
/>

---------

Co-authored-by: root <root@localhost.localdomain>
Co-authored-by: Kirk Loretz <kirk-loretz-fsn@users.noreply.github.com>
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

Author: 4 people

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Allow `@tailwindcss/cli` in `--watch` mode to use polling with `--poll` when filesystem events are unreliable or unavailable ([#20297](https://github.com/tailwindlabs/tailwindcss/pull/20297))
 - Canonicalization: match arbitrary hex colors against theme colors case-insensitively (e.g. `bg-[#fff]` and `bg-[#FFF]` → `bg-white`) ([#20298](https://github.com/tailwindlabs/tailwindcss/pull/20298))
+- Prevent Preflight from overriding Firefox's native `iframe:focus-visible` outline styles ([#20292](https://github.com/tailwindlabs/tailwindcss/pull/20292))
 
 ## [4.3.2] - 2026-06-26
 

integrations/cli/index.test.ts

@@ -2903,7 +2903,7 @@ test(
           border-color: inherit;
           border-collapse: collapse;
         }
-        :-moz-focusring {
+        :-moz-focusring:where(:not(iframe)) {
           outline: auto;
         }
         progress {

packages/@tailwindcss-postcss/src/__snapshots__/index.test.ts.snap

@@ -110,7 +110,7 @@ exports[`\`@import 'tailwindcss'\` is replaced with the generated CSS 1`] = `
     border-collapse: collapse;
   }
 
-  :-moz-focusring {
+  :-moz-focusring:where(:not(iframe)) {
     outline: auto;
   }
 

packages/tailwindcss/preflight.css

@@ -170,7 +170,7 @@ table {
   Use the modern Firefox focus style for all focusable elements.
 */
 
-:-moz-focusring {
+:-moz-focusring:where(:not(iframe)) {
   outline: auto;
 }
 

packages/tailwindcss/src/__snapshots__/index.test.ts.snap

@@ -226,7 +226,7 @@ exports[`compiling CSS > prefix all CSS variables inside preflight 1`] = `
     border-collapse: collapse;
   }
 
-  :-moz-focusring {
+  :-moz-focusring:where(:not(iframe)) {
     outline: auto;
   }
 

packages/tailwindcss/src/source-maps/source-map.test.ts

@@ -338,8 +338,8 @@ test('source maps trace back to @import location', async ({ expect }) => {
              ^^^^^^^^^^^^^^^^^^^^^^^^^ CS @ 73:4-29                                                      |        ^^^^^^^^^^^^^^^^^^^^^^^^^ CS @ 166:2-27
                                                                                                          | 167  }
      74    }                                                                                             | 
-     75    :-moz-focusring {                                                                             | 173  :-moz-focusring {
-           ^^^^^^^^^^^^^^^^ CT @ 75:2-18                                                                 |      ^^^^^^^^^^^^^^^^ CT @ 173:0-16
+     75    :-moz-focusring:where(:not(iframe)) {                                                         | 173  :-moz-focusring:where(:not(iframe)) {
+           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ CT @ 75:2-38                                             |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ CT @ 173:0-36
      76      outline: auto;                                                                              | 174    outline: auto;
              ^^^^^^^^^^^^^ CU @ 76:4-17                                                                  |        ^^^^^^^^^^^^^ CU @ 174:2-15
                                                                                                          | 175  }