diff --git a/src/Header.module.css b/src/Header.module.css
index 7f94581..857a73b 100644
--- a/src/Header.module.css
+++ b/src/Header.module.css
@@ -131,7 +131,7 @@
   }
 
   .leftNav h3 {
-    font-size: 18px;
+    font-size: var(--font-size-subtitle);
   }
 
   .nav {
diff --git a/src/ListBox.module.css b/src/ListBox.module.css
index ab17f3c..2b9cc2d 100644
--- a/src/ListBox.module.css
+++ b/src/ListBox.module.css
@@ -19,7 +19,7 @@
   padding: 8px 16px;
   outline: none;
   cursor: pointer;
-  font-size: 15px;
+  font-size: var(--font-size-body);
   min-height: 32px;
 }
 
diff --git a/src/Menu.module.css b/src/Menu.module.css
index 3ef29e6..110dd47 100644
--- a/src/Menu.module.css
+++ b/src/Menu.module.css
@@ -12,7 +12,10 @@
   align-items: center;
   padding: 0 12px;
   color: var(--primary-content);
-  font-size: 14px;
+  font-size: var(--font-size-body);
+  text-overflow: ellipsis;
+  overflow: hidden;
+  white-space: nowrap;
 }
 
 .menuItem > * {
diff --git a/src/Modal.module.css b/src/Modal.module.css
index 96bda6f..2def854 100644
--- a/src/Modal.module.css
+++ b/src/Modal.module.css
@@ -29,7 +29,7 @@
 
 .modalHeader h3 {
   font-weight: 600;
-  font-size: 24px;
+  font-size: var(--font-size-title);
   margin: 0;
 }
 
diff --git a/src/Tooltip.module.css b/src/Tooltip.module.css
index fccb4cb..8d7a877 100644
--- a/src/Tooltip.module.css
+++ b/src/Tooltip.module.css
@@ -8,7 +8,7 @@
   border-radius: 8px;
   max-width: 135px;
   width: max-content;
-  font-size: 12px;
+  font-size: var(--font-size-caption);
   font-weight: 500;
   text-align: center;
 }
diff --git a/src/UrlParams.ts b/src/UrlParams.ts
index 01ea548..ce90427 100644
--- a/src/UrlParams.ts
+++ b/src/UrlParams.ts
@@ -21,30 +21,60 @@ export interface UrlParams {
   roomAlias: string | null;
   roomId: string | null;
   viaServers: string[];
-  // Whether the app is running in embedded mode, and should keep the user
-  // confined to the current room
+  /**
+   * Whether the app is running in embedded mode, and should keep the user
+   * confined to the current room.
+   */
   isEmbedded: boolean;
-  // Whether the app should pause before joining the call until it sees an
-  // io.element.join widget action, allowing it to be preloaded
+  /**
+   * Whether the app should pause before joining the call until it sees an
+   * io.element.join widget action, allowing it to be preloaded.
+   */
   preload: boolean;
-  // Whether to hide the room header when in a call
+  /**
+   * Whether to hide the room header when in a call.
+   */
   hideHeader: boolean;
-  // Whether to hide the screen-sharing button
+  /**
+   * Whether to hide the screen-sharing button.
+   */
   hideScreensharing: boolean;
-  // Whether to start a walkie-talkie call instead of a video call
+  /**
+   * Whether to start a walkie-talkie call instead of a video call.
+   */
   isPtt: boolean;
-  // Whether to use end-to-end encryption
+  /**
+   * Whether to use end-to-end encryption.
+   */
   e2eEnabled: boolean;
-  // The user's ID (only used in matryoshka mode)
+  /**
+   * The user's ID (only used in matryoshka mode).
+   */
   userId: string | null;
-  // The display name to use for auto-registration
+  /**
+   * The display name to use for auto-registration.
+   */
   displayName: string | null;
-  // The device's ID (only used in matryoshka mode)
+  /**
+   * The device's ID (only used in matryoshka mode).
+   */
   deviceId: string | null;
-  // The base URL of the homeserver to use for media lookups in matryoshka mode
+  /**
+   * The base URL of the homeserver to use for media lookups in matryoshka mode.
+   */
   baseUrl: string | null;
-  // The BCP 47 code of the language the app should use
+  /**
+   * The BCP 47 code of the language the app should use.
+   */
   lang: string | null;
+  /**
+   * The fonts which the interface should use, if not empty.
+   */
+  fonts: string[];
+  /**
+   * The factor by which to scale the interface's font size.
+   */
+  fontScale: number | null;
 }
 
 /**
@@ -81,6 +111,8 @@ export const getUrlParams = (
       ? fragment
       : fragment.substring(0, fragmentQueryStart);
 
+  const fontScale = parseFloat(getParam("fontScale") ?? "");
+
   return {
     roomAlias: fragmentRoute.length > 1 ? fragmentRoute : null,
     roomId: getParam("roomId"),
@@ -96,6 +128,8 @@ export const getUrlParams = (
     deviceId: getParam("deviceId"),
     baseUrl: getParam("baseUrl"),
     lang: getParam("lang"),
+    fonts: getAllParams("font"),
+    fontScale: Number.isNaN(fontScale) ? null : fontScale,
   };
 };
 
diff --git a/src/UserMenu.module.css b/src/UserMenu.module.css
index 2a0ed94..06e817c 100644
--- a/src/UserMenu.module.css
+++ b/src/UserMenu.module.css
@@ -10,13 +10,13 @@
 .avatar {
   width: 24px;
   height: 24px;
-  font-size: 12px;
+  font-size: var(--font-size-caption);
 }
 
 @media (min-width: 800px) {
   .avatar {
     width: 32px;
     height: 32px;
-    font-size: 15px;
+    font-size: var(--font-size-body);
   }
 }
diff --git a/src/auth/LoginPage.module.css b/src/auth/LoginPage.module.css
index 0d135c3..4a798d7 100644
--- a/src/auth/LoginPage.module.css
+++ b/src/auth/LoginPage.module.css
@@ -36,7 +36,7 @@
 
 .formContainer h4 {
   font-weight: normal;
-  font-size: 18px;
+  font-size: var(--font-size-subtitle);
   margin-bottom: 0;
 }
 
@@ -48,7 +48,7 @@
 .formContainer button {
   height: 48px;
   width: 100%;
-  font-size: 15px;
+  font-size: var(--font-size-body);
   font-weight: 600;
 }
 
@@ -61,7 +61,7 @@
 
 .authLinks {
   margin-bottom: 100px;
-  font-size: 15px;
+  font-size: var(--font-size-body);
 }
 
 .authLinks a {
diff --git a/src/button/Button.module.css b/src/button/Button.module.css
index 53e3b2d..eb8f0b1 100644
--- a/src/button/Button.module.css
+++ b/src/button/Button.module.css
@@ -41,7 +41,7 @@ limitations under the License.
 .copyButton {
   padding: 7px 15px;
   border-radius: 8px;
-  font-size: 14px;
+  font-size: var(--font-size-body);
   font-weight: 700;
 }
 
@@ -142,7 +142,7 @@ limitations under the License.
 
 .copyButton span {
   font-weight: 600;
-  font-size: 15px;
+  font-size: var(--font-size-body);
   margin-right: 10px;
   overflow: hidden;
   white-space: nowrap;
diff --git a/src/index.css b/src/index.css
index 6e9a912..fbc9d1a 100644
--- a/src/index.css
+++ b/src/index.css
@@ -23,8 +23,20 @@ limitations under the License.
 @import "normalize.css/normalize.css";
 
 :root {
+  --font-family: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI",
+    "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans",
+    "Helvetica Neue", sans-serif;
   --inter-unicode-range: U+0000-20e2, U+20e4-23ce, U+23d0-24c1, U+24c3-259f,
     U+25c2-2664, U+2666-2763, U+2765-2b05, U+2b07-2b1b, U+2b1d-10FFFF;
+
+  --font-scale: 1;
+  --font-size-micro: calc(10px * var(--font-scale));
+  --font-size-caption: calc(12px * var(--font-scale));
+  --font-size-body: calc(15px * var(--font-scale));
+  --font-size-subtitle: calc(18px * var(--font-scale));
+  --font-size-title: calc(24px * var(--font-scale));
+  --font-size-headline: calc(32px * var(--font-scale));
+
   --accent: #0dbd8b;
   --accent-20: #0dbd8b33;
   --alert: #ff5b55;
@@ -127,9 +139,7 @@ body {
   color: var(--primary-content);
   color-scheme: dark;
   margin: 0;
-  font-family: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto",
-    "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue",
-    sans-serif;
+  font-family: var(--font-family);
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
 }
@@ -159,28 +169,31 @@ a {
 /* Headline Semi Bold */
 h1 {
   font-weight: 600;
-  font-size: 32px;
-  line-height: 39px;
+  font-size: var(--font-size-headline);
 }
 
 /* Title */
 h2 {
   font-weight: 600;
-  font-size: 24px;
-  line-height: 29px;
+  font-size: var(--font-size-title);
 }
 
 /* Subtitle */
 h3 {
   font-weight: 400;
-  font-size: 18px;
-  line-height: 22px;
+  font-size: var(--font-size-subtitle);
+}
+
+h1,
+h2,
+h3 {
+  line-height: 1.2;
 }
 
 /* Body */
 p {
-  font-size: 15px;
-  line-height: 24px;
+  font-size: var(--font-size-body);
+  line-height: var(--font-size-title);
 }
 
 a {
@@ -202,21 +215,21 @@ hr {
   text-align: center;
   height: 5px;
   font-weight: 600;
-  font-size: 15px;
+  font-size: var(--font-size-body);
   line-height: 24px;
   margin: 0 12px;
 }
 
 summary {
-  font-size: 14px;
+  font-size: var(--font-size-body);
 }
 
 details > :not(summary) {
-  margin-left: 16px;
+  margin-left: var(--font-size-body);
 }
 
 details[open] > summary {
-  margin-bottom: 16px;
+  margin-bottom: var(--font-size-body);
 }
 
 #root > [data-overlay-container] {
diff --git a/src/initializer.tsx b/src/initializer.tsx
index 65183b0..6290764 100644
--- a/src/initializer.tsx
+++ b/src/initializer.tsx
@@ -133,6 +133,21 @@ export class Initializer {
         import.meta.env.VITE_THEME_BACKGROUND_85 as string
       );
     }
+
+    // Custom fonts
+    const { fonts, fontScale } = getUrlParams();
+    if (fontScale !== null) {
+      document.documentElement.style.setProperty(
+        "--font-scale",
+        fontScale.toString()
+      );
+    }
+    if (fonts.length > 0) {
+      document.documentElement.style.setProperty(
+        "--font-family",
+        fonts.map((f) => `"${f}"`).join(", ")
+      );
+    }
   }
 
   public static init(): Promise<void> | null {
diff --git a/src/input/Input.module.css b/src/input/Input.module.css
index c04a85a..fce0a93 100644
--- a/src/input/Input.module.css
+++ b/src/input/Input.module.css
@@ -32,7 +32,7 @@
 .inputField input,
 .inputField textarea {
   font-weight: 400;
-  font-size: 15px;
+  font-size: var(--font-size-body);
   border: none;
   border-radius: 4px;
   padding: 12px 9px 10px 9px;
@@ -73,7 +73,7 @@
     top 0.25s ease-out 0.1s, background-color 0.25s ease-out 0.1s;
   color: var(--tertiary-content);
   background-color: transparent;
-  font-size: 15px;
+  font-size: var(--font-size-body);
   position: absolute;
   left: 0;
   top: 0;
@@ -104,7 +104,7 @@
   background-color: var(--system);
   transition: font-size 0.25s ease-out 0s, color 0.25s ease-out 0s,
     top 0.25s ease-out 0s, background-color 0.25s ease-out 0s;
-  font-size: 10px;
+  font-size: var(--font-size-micro);
   top: -13px;
   padding: 0 2px;
   pointer-events: auto;
@@ -125,7 +125,7 @@
   display: flex;
   align-items: center;
   flex-grow: 1;
-  font-size: 15px;
+  font-size: var(--font-size-body);
   line-height: 24px;
 }
 
@@ -174,7 +174,7 @@
 
 .errorMessage {
   margin: 0;
-  font-size: 13px;
+  font-size: var(--font-size-caption);
   color: var(--alert);
   font-weight: 600;
 }
diff --git a/src/input/SelectInput.module.css b/src/input/SelectInput.module.css
index f1312c8..afa0ce4 100644
--- a/src/input/SelectInput.module.css
+++ b/src/input/SelectInput.module.css
@@ -7,7 +7,7 @@
 
 .label {
   font-weight: 600;
-  font-size: 18px;
+  font-size: var(--font-size-subtitle);
   margin-top: 0;
   margin-bottom: 12px;
 }
@@ -20,7 +20,7 @@
   background-color: var(--background);
   border-radius: 8px;
   border: 1px solid var(--quinary-content);
-  font-size: 15px;
+  font-size: var(--font-size-body);
   color: var(--primary-content);
   height: 40px;
   max-width: 100%;
diff --git a/src/room/GroupCallInspector.module.css b/src/room/GroupCallInspector.module.css
index 289523f..74dc8e9 100644
--- a/src/room/GroupCallInspector.module.css
+++ b/src/room/GroupCallInspector.module.css
@@ -19,7 +19,7 @@
 }
 
 .sequenceDiagramViewer :global(.messageText) {
-  font-size: 12px;
+  font-size: var(--font-size-caption);
   fill: var(--primary-content) !important;
   stroke: var(--primary-content) !important;
 }
diff --git a/src/room/LobbyView.module.css b/src/room/LobbyView.module.css
index 03efa7d..55b7b5b 100644
--- a/src/room/LobbyView.module.css
+++ b/src/room/LobbyView.module.css
@@ -54,7 +54,7 @@ limitations under the License.
   bottom: 86px;
   left: 50%;
   font-weight: 600;
-  font-size: 15px;
+  font-size: var(--font-size-body);
   transform: translateX(-50%);
 }
 
diff --git a/src/room/PTTCallView.module.css b/src/room/PTTCallView.module.css
index f053850..834840c 100644
--- a/src/room/PTTCallView.module.css
+++ b/src/room/PTTCallView.module.css
@@ -64,7 +64,7 @@
 .actionTip {
   margin-top: 20px;
   margin-bottom: 20px;
-  font-size: 17px;
+  font-size: var(--font-size-subtitle);
 }
 
 .footer {
diff --git a/src/typography/Typography.module.css b/src/typography/Typography.module.css
index a22f22e..451f2d9 100644
--- a/src/typography/Typography.module.css
+++ b/src/typography/Typography.module.css
@@ -1,11 +1,11 @@
 .caption {
-  font-size: 12px;
-  line-height: 15px;
+  font-size: var(--font-size-caption);
+  line-height: var(--font-size-body);
 }
 
 .micro {
-  font-size: 10px;
-  line-height: 12px;
+  font-size: var(--font-size-micro);
+  line-height: var(--font-size-caption);
 }
 
 .regular {
diff --git a/src/video-grid/VideoTile.module.css b/src/video-grid/VideoTile.module.css
index ec496e6..94f53a7 100644
--- a/src/video-grid/VideoTile.module.css
+++ b/src/video-grid/VideoTile.module.css
@@ -119,9 +119,9 @@
 }
 
 .memberName span {
-  font-size: 12px;
+  font-size: var(--font-size-caption);
   font-weight: 400;
-  line-height: 16px;
+  line-height: var(--font-size-body);
   text-overflow: ellipsis;
   overflow: hidden;
   white-space: nowrap;
@@ -152,8 +152,8 @@
   align-items: center;
   padding: 4px 8px;
   font-weight: normal;
-  font-size: 12px;
-  line-height: 15px;
+  font-size: var(--font-size-caption);
+  line-height: var(--font-size-body);
 }
 
 .screensharePIP {
diff --git a/test/initializer-test.ts b/test/initializer-test.ts
new file mode 100644
index 0000000..b31f6a8
--- /dev/null
+++ b/test/initializer-test.ts
@@ -0,0 +1,33 @@
+/*
+Copyright 2022 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { Initializer } from "../src/initializer";
+
+test("initBeforeReact sets font family from URL param", () => {
+  window.location.hash = "#?font=DejaVu Sans";
+  Initializer.initBeforeReact();
+  expect(
+    getComputedStyle(document.documentElement).getPropertyValue("--font-family")
+  ).toBe('"DejaVu Sans"');
+});
+
+test("initBeforeReact sets font scale from URL param", () => {
+  window.location.hash = "#?fontScale=1.2";
+  Initializer.initBeforeReact();
+  expect(
+    getComputedStyle(document.documentElement).getPropertyValue("--font-scale")
+  ).toBe("1.2");
+});