New: IE flexbox compat hint

Hint that warns users when they do configure IE as a target browser and
flexbox is used in one of the stylesheets.

- - - - - - - - - - - - - - - - - - - -
 
Fix: #4072
Close #4073

packages/hint-ie-flexbox-compat/.npmrc

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+package-lock=false

packages/hint-ie-flexbox-compat/LICENSE.txt

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packages/hint-ie-flexbox-compat/README.md

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+# Internet Explorer Flexbox Compatibility (`ie-flexbox-compat`)
+
+`ie-flexbox-compat` warns about potential CSS Flexbox compatibility bugs with
+Internet Explorer.
+
+## Why is this important?
+
+Internet Explorer 11 and older suffer from a number of CSS Flexbox
+compatibility bugs.\
+In some cases, the CSS Flexbox specification changed after Internet Explorer
+implemented it, in others, the implementation was just incorrect.
+
+Because of this, the layout on your site may be different between Internet
+Explorer and other browsers if you use Flexbox.\
+The size or alignment of elements may be incorrect or content may overflow.
+
+As described in
+[Microsoft's support policy for Internet Explorer][IE support policy], support
+for older versions of this browser ended and Microsoft no longer provides
+security updates or technical support for these versions.\
+It is therefore discouraged to introduce flexbox if you still need to target
+IE.
+
+You can find more information about each individual bug as well workarounds for
+them on the [Flexbugs repository][flexbugs].
+
+## What does the hint check?
+
+This hint checks two things:
+
+* That the configured target browsers include Internet Explorer.
+* And that CSS on your site uses flexbox.
+
+### Examples that **trigger** the hint
+
+If your site declares a flexbox container by using:
+
+```css
+#container {
+    display: flex;
+}
+```
+
+Or by using:
+
+```css
+#container {
+    display: inline-flex;
+}
+```
+
+## How to use this hint?
+
+This package is installed automatically by webhint:
+
+```bash
+npm install hint --save-dev
+```
+
+To use it, activate it via the [`.hintrc`][hintrc] configuration file:
+
+```json
+{
+    "connector": {...},
+    "formatters": [...],
+    "hints": {
+        "ie-flexbox-compat": "warning",
+        ...
+    },
+    "parsers": [...],
+    ...
+}
+```
+
+**Note**: The recommended way of running webhint is as a `devDependency` of
+your project.
+
+[hintrc]: https://webhint.io/docs/user-guide/configuring-webhint/summary/
+[IE support policy]: https://www.microsoft.com/en-us/microsoft-365/windows/end-of-ie-support
+[flexbugs]: https://github.com/philipwalton/flexbugs

packages/hint-ie-flexbox-compat/package.json

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+{
+  "ava": {
+    "failFast": false,
+    "files": [
+      "dist/tests/**/*.js",
+      "!dist/tests/**/fixtures/**/*.js"
+    ],
+    "timeout": "1m"
+  },
+  "dependencies": {
+    "@hint/utils-i18n": "^1.0.5",
+    "@hint/utils-types": "^1.1.0",
+    "@hint/utils-fs": "^1.0.5",
+    "@hint/utils-css": "^1.0.5",
+    "@hint/parser-css": "^3.0.21"
+  },
+  "description": "hint that informs about potential flexbox compatibility bugs with Internet Explorer",
+  "devDependencies": {
+    "@hint/utils-create-server": "^3.4.6",
+    "@hint/utils-tests-helpers": "^6.3.0",
+    "@types/node": "^14.11.2",
+    "@typescript-eslint/eslint-plugin": "^4.0.1",
+    "@typescript-eslint/parser": "^4.0.1",
+    "ava": "^3.12.1",
+    "cpx": "^1.5.0",
+    "eslint": "^7.8.1",
+    "eslint-plugin-import": "^2.22.0",
+    "eslint-plugin-markdown": "^1.0.2",
+    "npm-run-all": "^4.1.5",
+    "nyc": "^15.1.0",
+    "rimraf": "^3.0.2",
+    "typescript": "^4.0.2"
+  },
+  "files": [
+    "dist/src"
+  ],
+  "homepage": "https://webhint.io/",
+  "keywords": [
+    "hint",
+    "ie-flexbox-compat",
+    "ie-flexbox-compat-hint"
+  ],
+  "license": "Apache-2.0",
+  "main": "dist/src/hint.js",
+  "name": "@hint/hint-ie-flexbox-compat",
+  "nyc": {
+    "extends": "../../.nycrc"
+  },
+  "peerDependencies": {
+    "hint": "^6.0.0"
+  },
+  "private": true,
+  "repository": "webhintio/hint",
+  "scripts": {
+    "build": "npm run i18n && npm-run-all build:*",
+    "build-release": "npm run clean && npm run i18n && npm run build:assets && tsc --inlineSourceMap false --removeComments true",
+    "build:assets": "cpx \"./{src,tests}/**/{!(*.ts),.!(ts)}\" dist",
+    "build:ts": "tsc -b",
+    "clean": "rimraf dist",
+    "i18n": "node ../../scripts/create-i18n.js",
+    "lint": "npm-run-all lint:*",
+    "lint:js": "eslint . --cache --ext .js,.md,.ts --ignore-path ../../.eslintignore",
+    "lint:dependencies": "node ../../scripts/lint-dependencies.js",
+    "lint:md": "node ../../scripts/lint-markdown.js",
+    "test": "npm run i18n && npm run lint && npm run build && npm run test-only",
+    "test-only": "nyc ava",
+    "test-release": "npm run i18n && npm run lint && npm run build-release && ava",
+    "init": "npm install && npm run build",
+    "watch": "npm run build && npm-run-all --parallel -c watch:*",
+    "watch:assets": "npm run build:assets -- -w --no-initial",
+    "watch:test": "ava --watch",
+    "watch:ts": "npm run build:ts -- --watch"
+  },
+  "version": "0.0.1"
+}

packages/hint-ie-flexbox-compat/src/_locales/en/messages.json

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+{
+    "description": {
+        "description": "Metadata description",
+        "message": "Informs users that Internet Explorer suffers from Flexbox compatibility bugs."
+    },
+    "name": {
+        "description": "Metadata name",
+        "message": "IE Flexbox Compatibility"
+    },
+    "usingFlexWithIE": {
+        "description": "String warning users that flex is used and IE is a target browser",
+        "message": "CSS Flexbox may render differently in Internet Explorer than more recent browsers. Check this page in IE then see the documentation for workarounds if needed."
+    }
+}