Simplify font family settings

Before we removed compatibility mode, the font-families file included multiple ‘settings’ (realistically, more like constants) that defined the font to be used depending on whether compatibility mode was enabled or not.

Now that we only have one possible default font stack, we can remove the unnecessary abstraction.

We also need to update the default value for the `$govuk-include-default-font-face` setting. Previously it checked whether the chosen font family was the default stack based on GDS Transport, but we can instead just check whether “GDS Transport” is in the font list. This also makes it more robust if somebody for example wants to update the setting to change the fallback fonts.

packages/govuk-frontend/src/govuk/settings/_all.scss

@@ -15,7 +15,6 @@
 @import "spacing";
 @import "measurements";
 
-@import "typography-font-families";
 @import "typography-font";
 @import "typography-responsive";
 

packages/govuk-frontend/src/govuk/settings/_typography-font.scss

@@ -1,5 +1,3 @@
-@import "typography-font-families";
-
 ////
 /// @group settings/typography
 ////
@@ -13,7 +11,7 @@
 /// @type List
 /// @access public
 
-$govuk-font-family: $govuk-font-family-gds-transport !default;
+$govuk-font-family: "GDS Transport", arial, sans-serif !default;
 
 /// Font families to use for print media
 ///
@@ -27,15 +25,13 @@ $govuk-font-family-print: sans-serif !default;
 
 /// Include the default @font-face declarations
 ///
-/// If you have set $govuk-font-family to something other than
-/// `$govuk-font-family-gds-transport` this option is disabled by default.
+/// Defaults to true if "GDS Transport" appears in the $govuk-font-family
+/// setting.
 ///
 /// @type Boolean
 /// @access public
 
-$govuk-include-default-font-face: (
-  $govuk-font-family == $govuk-font-family-gds-transport
-) !default;
+$govuk-include-default-font-face: if(index($govuk-font-family, "GDS Transport"), true, false) !default;
 
 // =========================================================
 // Font weights

packages/govuk-frontend/src/govuk/settings/typography.test.js

@@ -0,0 +1,44 @@
+const { compileSassString } = require('govuk-frontend-helpers/tests')
+
+describe('$govuk-include-default-font-face', () => {
+  it('is true if $govuk-font-family is default', async () => {
+    const sass = `
+      @import "settings/typography-font";
+      :root {
+        --result: #{$govuk-include-default-font-face}
+      }
+    `
+
+    await expect(compileSassString(sass)).resolves.toMatchObject({
+      css: expect.stringContaining('--result: true')
+    })
+  })
+
+  it('is true if $govuk-font-family includes GDS Transport', async () => {
+    const sass = `
+    $govuk-font-family: "GDS Transport", "Comic Sans MS", "Comic Sans", cursive;
+      @import "settings/typography-font";
+      :root {
+        --result: #{$govuk-include-default-font-face}
+      }
+    `
+
+    await expect(compileSassString(sass)).resolves.toMatchObject({
+      css: expect.stringContaining('--result: true')
+    })
+  })
+
+  it('is false if $govuk-font-family does not include GDS Transport', async () => {
+    const sass = `
+    $govuk-font-family: "Comic Sans MS", "Comic Sans", cursive;
+      @import "settings/typography-font";
+      :root {
+        --result: #{$govuk-include-default-font-face}
+      }
+    `
+
+    await expect(compileSassString(sass)).resolves.toMatchObject({
+      css: expect.stringContaining('--result: false')
+    })
+  })
+})