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

Issue number: resolves #24638, resolves #18592

---------

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issue. -->

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etc). Submit multiple pull requests if needed. -->

## 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/css/core.scss

@@ -13,6 +13,7 @@ html.md {
 }
 
 html {
+  --ion-default-dynamic-font: -apple-system-body;
   --ion-font-family: var(--ion-default-font);
 }
 

core/src/css/test/a11y/typography.e2e.ts

@@ -0,0 +1,35 @@
+import { expect } from '@playwright/test';
+import { configs, test } from '@utils/test/playwright';
+
+configs({ directions: ['ltr'], modes: ['ios'] }).forEach(({ title, screenshot, config }) => {
+  test.describe(title('typography: font scaling'), () => {
+    test('should scale text on larger font sizes', async ({ page }) => {
+      await page.setContent(
+        `
+        <style>
+          html {
+            font-size: 310%;
+          }
+        </style>
+
+        <div>
+          <h1>Heading 1</h1>
+          <h2>Heading 2</h2>
+          <h3>Heading 3</h3>
+          <h4>Heading 4</h4>
+          <h5>Heading 5</h5>
+          <h6>Heading 6</h6>
+          <p>
+            The quick brown fox <ion-text><sup>jumps</sup></ion-text> over the <ion-text><sub>lazy dog</sub></ion-text>
+          </p>
+        </div>
+      `,
+        config
+      );
+
+      const div = page.locator('div');
+
+      await expect(div).toHaveScreenshot(screenshot(`typography-scale`));
+    });
+  });
+});

core/src/css/typography.scss

@@ -11,27 +11,40 @@ $headings-font-weight:         500 !default;
 $headings-line-height:         1.2 !default;
 
 /// @prop - Font size of heading level 1
-$h1-font-size:                 26px !default;
+$h1-font-size:                 dynamic-font(26px) !default;
 
 /// @prop - Font size of heading level 2
-$h2-font-size:                 24px !default;
+$h2-font-size:                 dynamic-font(24px) !default;
 
 /// @prop - Font size of heading level 3
-$h3-font-size:                 22px !default;
+$h3-font-size:                 dynamic-font(22px) !default;
 
 /// @prop - Font size of heading level 4
-$h4-font-size:                 20px !default;
+$h4-font-size:                 dynamic-font(20px) !default;
 
 /// @prop - Font size of heading level 5
-$h5-font-size:                 18px !default;
+$h5-font-size:                 dynamic-font(18px) !default;
 
 /// @prop - Font size of heading level 6
-$h6-font-size:                 16px !default;
+$h6-font-size:                 dynamic-font(16px) !default;
 
 html {
   font-family: var(--ion-font-family);
 }
 
+/**
+ * Dynamic Type is an iOS-only feature, so
+ * this should only be enabled on iOS devices.
+ */
+@supports (-webkit-touch-callout: none) {
+  html {
+    /**
+     * Includes fallback if Dynamic Type is not enabled.
+     */
+    font: var(--ion-dynamic-font, 16px var(--ion-font-family));
+  }
+}
+
 a {
   background-color: transparent;
   color: ion-color(primary, base);