feat(a11y): add dynamic font scaling (#28314)

Issue number: resolves #24638, resolves #18592

---------

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issue. -->

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

Developers have requested that Ionic Framework support the dynamic type
feature on iOS for accessibility purposes. Ionic applications do not
respond to font scaling on iOS which can create inaccessible
applications particularly for users with low vision. Ionic apps on
Android devices currently support the Android equivalent due to
functionality in the Chromium webview.

Developers have also requested a way of adjusting the fonts in their
Ionic UI components consistently.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Ionic components now use `rem` instead of `px` where appropriate. This
means devs can change the font size on `html` and the text in supported
Ionic components will scale up/down appropriately
- Add support for Dynamic Type on iOS (the iOS version of Dynamic Font
Scaling)

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

<!-- If this introduces a breaking change, please describe the impact
and migration path for existing applications below. -->


## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

---------

Co-authored-by: Maria Hutt <thetaPC@users.noreply.github.com>
Co-authored-by: Brandy Carney <brandyscarney@users.noreply.github.com>
Co-authored-by: Shawn Taylor <shawn@ionic.io>
Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Sean Perkins <sean@ionic.io>
Co-authored-by: Sean Perkins <13732623+sean-perkins@users.noreply.github.com>
Co-authored-by: Amanda Johnston <90629384+amandaejohnston@users.noreply.github.com>

core/src/components/back-button/back-button.ios.scss

@@ -13,10 +13,26 @@
   --color: #{$back-button-ios-color};
   --icon-margin-end: 1px;
   --icon-margin-start: -4px;
+  /**
+   * The icon should be sized relative
+   * to the size of the text which is
+   * why we use em here instead of rem.
+   * This allows developers to override
+   * the text font size while ensuring that
+   * the icon is sized relative to that.
+   */
   --icon-font-size: 1.6em;
   --min-height: 32px;
 
-  font-size: 17px;
+  /**
+   * Main content should be prioritized over the back
+   * button which is why a maximum font size is applied.
+   * Also, we want the text to remain readable
+   * so a minimum font size is applied.
+   * Using 1.294 instead of 1.3 aligns the text
+   * with the icon a bit nicer in Firefox.
+   */
+  font-size: dynamic-font-clamp(1, 17px, 1.294);
 }
 
 .button-native {

core/src/components/back-button/back-button.md.scss

@@ -13,14 +13,14 @@
   --color: #{$back-button-md-color};
   --icon-margin-end: 0;
   --icon-margin-start: 0;
-  --icon-font-size: 24px;
+  --icon-font-size: #{dynamic-font(24px)};
   --icon-font-weight: normal;
   --min-height: 32px;
   --min-width: 44px;
   --padding-start: 12px;
   --padding-end: 12px;
 
-  font-size: 14px;
+  font-size: dynamic-font(14px);
   font-weight: 500;
 
   text-transform: uppercase;
@@ -30,7 +30,14 @@
   --border-radius: 50%;
 
   min-width: 48px;
-  height: 48px;
+  min-height: 48px;
+
+  /**
+   * This allows the icon only button to
+   * keep its circular shape even when the
+   * text scales up.
+   */
+  aspect-ratio: 1 / 1;
 }
 
 .button-native {

core/src/components/back-button/test/a11y/back-button.e2e.ts

@@ -0,0 +1,29 @@
+import { expect } from '@playwright/test';
+import { configs, test } from '@utils/test/playwright';
+
+configs({ directions: ['ltr'] }).forEach(({ title, screenshot, config }) => {
+  test.describe(title('back-button: font scaling'), () => {
+    test('should scale text on larger font sizes', async ({ page }) => {
+      await page.setContent(
+        `
+        <style>
+          html {
+            font-size: 36px;
+          }
+        </style>
+
+        <ion-toolbar>
+          <ion-buttons slot="start">
+            <ion-back-button default-href="/" text="Back"></ion-back-button>
+          </ion-buttons>
+        </ion-toolbar>
+      `,
+        config
+      );
+
+      const backButton = page.locator('ion-back-button');
+
+      await expect(backButton).toHaveScreenshot(screenshot(`back-button-scale`));
+    });
+  });
+});