diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5ca469cffd..350599c08a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,8 +21,20 @@ Items that are related, such as breaking changes, new features, or changes to ex
 
 ## [Unreleased]
 
+### Breaking
+
+- [@siteimprove/alfa-predicate](packages/alfa-predicate): The `Predicate<T>` type now no longer duals as a refinement type. This behavior has instead been split out into the new `Refinement<T, U extends T>` type. This also means that none of the functions on the `Predicate` namespace will perform type narrowing. Instead, an equivalent function on the `Refinement` namespace should be used if type narrowing is desired. ([#415](../../pull/415))
+
+- [@siteimprove/alfa-http](packages/alfa-http): `Request#url` and `Response#url` now return `URL` instances rather than strings. ([#421](../../pull/421))
+
+- [@siteimprove/alfa-frontier](packages/alfa-frontier), [@siteimprove/alfa-scraper](packages/alfa-scraper): `Frontier` and `Scraper` now work on `URL` instances from the @siteimprove/alfa-url package rather than WHATWG URL instances. ([#421](../../pull/421))
+
 ### Added
 
+- [@siteimprove/alfa-refinement](packages/alfa-refinement): A new package has been added with functionality for working with refinement types, also known as type guards in TypeScript. ([#415](../../pull/415))
+
+- [@siteimprove/alfa-url](packages/alfa-url): A new package has been added with an implementation of an immutable URL representation. ([#421](../../pull/421))
+
 - [@siteimprove/alfa-result](packages/alfa-result): `Result` now provides an implementation of `Hashable`.
 
 - [@siteimprove/alfa-iterable](packages/alfa-iterable): `Iterable#takeLastWhile()`, `Iterable#takeLastUntil()`, `Iterable#skipLastWhile()`, `Iterable#skipLastUntil()`, `Iterable#trimLeading()`, `Iterable#trimTrailing()`, `Iterable#trim()`, and `Iterable#hash()` are now available.
@@ -33,10 +45,14 @@ Items that are related, such as breaking changes, new features, or changes to ex
 
 - [@siteimprove/alfa-sequence](packages/alfa-sequence): `Nil#hash()` now writes a size of 0 to prevent some avoidable collisions.
 
+- [@siteimprove/alfa-rules](packages/alfa-rules): SIA-R57 now only applies when at least one landmark is found one the page. ([#414](../../pull/414))
+
 ### Fixed
 
 - [@siteimprove/alfa-rules](packages/alfa-rules): SIA-R14 now only applies to focusable widgets with either an `aria-label` or `aria-labelledby` attribute, as intended. ([#409](../../pull/409))
 
+- [@siteimprove/alfa-rules](packages/alfa-rules): SIA-R68 now correctly excludes descendants of containers with an `aria-busy=true` attribute. ([#418](../../pull/418))
+
 - [@siteimprove/alfa-thunk](packages/alfa-thunk): `Thunk.flatMap()` now correctly defers evaluation of the passed thunk.
 
 ## [0.5.0](../../compare/v0.4.1...v0.5.0) (2020-09-22)
diff --git a/docs/examples/custom-testing/crawling/test.ts b/docs/examples/custom-testing/crawling/test.ts
index d1f3863631..739103bf97 100644
--- a/docs/examples/custom-testing/crawling/test.ts
+++ b/docs/examples/custom-testing/crawling/test.ts
@@ -25,9 +25,9 @@ Crawler.with(async (crawler) => {
 
       const earl = outcomes.map((outcome) => outcome.toEARL());
 
-      const url = new URL(input.response.url);
+      const { url } = input.response;
 
-      console.group(url.href);
+      console.group(url.toString());
       logStats(outcomes);
       console.groupEnd();
 
@@ -35,8 +35,8 @@ Crawler.with(async (crawler) => {
         path.join(
           __dirname,
           "outcomes",
-          url.host,
-          url.pathname.replace(/\/$/, "")
+          url.host.get(),
+          ...url.path.filter((segment) => segment !== "")
         ) + ".json";
 
       fs.mkdirSync(path.dirname(file), { recursive: true });
diff --git a/docs/examples/custom-testing/scraping/test.ts b/docs/examples/custom-testing/scraping/test.ts
index cc282978bd..d1d6658757 100644
--- a/docs/examples/custom-testing/scraping/test.ts
+++ b/docs/examples/custom-testing/scraping/test.ts
@@ -21,9 +21,9 @@ Scraper.with(async (scraper) => {
 
     const earl = outcomes.map((outcome) => outcome.toEARL());
 
-    const url = new URL(input.response.url);
+    const { url } = input.response;
 
-    console.group(url.href);
+    console.group(url.toString());
     logStats(outcomes);
     console.groupEnd();
 
@@ -31,8 +31,8 @@ Scraper.with(async (scraper) => {
       path.join(
         __dirname,
         "outcomes",
-        url.host,
-        url.pathname.replace(/\/$/, "")
+        url.host.get(),
+        ...url.path.filter((segment) => segment !== "")
       ) + ".json";
 
     fs.mkdirSync(path.dirname(file), { recursive: true });
diff --git a/packages/alfa-cli/bin/alfa/command/scrape/run.ts b/packages/alfa-cli/bin/alfa/command/scrape/run.ts
index 61010256bc..f6587d575c 100644
--- a/packages/alfa-cli/bin/alfa/command/scrape/run.ts
+++ b/packages/alfa-cli/bin/alfa/command/scrape/run.ts
@@ -15,6 +15,7 @@ import {
   Screenshot,
 } from "@siteimprove/alfa-scraper";
 import { Timeout } from "@siteimprove/alfa-time";
+import { URL } from "@siteimprove/alfa-url";
 
 import type { Arguments } from "./arguments";
 import type { Flags } from "./flags";
@@ -139,7 +140,7 @@ export const run: Command.Runner<typeof Flags, typeof Arguments> = async ({
   const timeout = Timeout.of(flags.timeout);
 
   const result = await scraper.scrape(
-    new URL(target, url.pathToFileURL(process.cwd() + path.sep)),
+    URL.parse(target, url.pathToFileURL(process.cwd() + path.sep).href).get(),
     {
       timeout,
       awaiter,
diff --git a/packages/alfa-cli/package.json b/packages/alfa-cli/package.json
index a7b038a180..3e5ea37034 100644
--- a/packages/alfa-cli/package.json
+++ b/packages/alfa-cli/package.json
@@ -35,6 +35,7 @@
     "@siteimprove/alfa-rules": "^0.5.0",
     "@siteimprove/alfa-scraper": "^0.5.0",
     "@siteimprove/alfa-time": "^0.5.0",
+    "@siteimprove/alfa-url": "^0.5.0",
     "@siteimprove/alfa-web": "^0.5.0",
     "@siteimprove/alfa-xpath": "^0.5.0",
     "@types/node": "^14.0.12",
diff --git a/packages/alfa-cli/tsconfig.json b/packages/alfa-cli/tsconfig.json
index 3c58e64109..7261b536af 100644
--- a/packages/alfa-cli/tsconfig.json
+++ b/packages/alfa-cli/tsconfig.json
@@ -90,6 +90,9 @@
     {
       "path": "../alfa-time"
     },
+    {
+      "path": "../alfa-url"
+    },
     {
       "path": "../alfa-web"
     },
diff --git a/packages/alfa-crawler/package.json b/packages/alfa-crawler/package.json
index 8fc208508a..88258759c1 100644
--- a/packages/alfa-crawler/package.json
+++ b/packages/alfa-crawler/package.json
@@ -23,6 +23,7 @@
     "@siteimprove/alfa-mapper": "^0.5.0",
     "@siteimprove/alfa-result": "^0.5.0",
     "@siteimprove/alfa-scraper": "^0.5.0",
+    "@siteimprove/alfa-url": "^0.5.0",
     "@siteimprove/alfa-web": "^0.5.0"
   },
   "devDependencies": {
diff --git a/packages/alfa-crawler/src/crawler.ts b/packages/alfa-crawler/src/crawler.ts
index 5466ee4fd3..8480165e87 100644
--- a/packages/alfa-crawler/src/crawler.ts
+++ b/packages/alfa-crawler/src/crawler.ts
@@ -3,6 +3,7 @@ import { Frontier } from "@siteimprove/alfa-frontier";
 import { Mapper } from "@siteimprove/alfa-mapper";
 import { Result } from "@siteimprove/alfa-result";
 import { Scraper, Screenshot } from "@siteimprove/alfa-scraper";
+import { URL } from "@siteimprove/alfa-url";
 import { Page } from "@siteimprove/alfa-web";
 
 const { isElement } = Element;
@@ -19,10 +20,12 @@ export class Crawler {
     scraper?: Promise<Scraper>
   ): Promise<T> {
     const crawler = await this.of(scraper);
-    const result = await mapper(crawler);
 
-    await crawler.close();
-    return result;
+    try {
+      return await mapper(crawler);
+    } finally {
+      await crawler.close();
+    }
   }
 
   private readonly _scraper: Scraper;
@@ -53,8 +56,8 @@ export class Crawler {
         frontier.complete(url);
 
         for (const page of result) {
-          if (page.response.url !== url.href) {
-            frontier.redirect(url.href, page.response.url);
+          if (!page.response.url.equals(url)) {
+            frontier.redirect(url, page.response.url);
           }
 
           for (const url of urls(page)) {
@@ -94,7 +97,7 @@ function* urls(page: Page): Iterable<URL> {
     if (isElement(node) && node.name === "a") {
       yield* node
         .attribute("href")
-        .map((href) => new URL(href.value, page.response.url));
+        .map((href) => URL.parse(href.value, page.response.url).get());
     }
   }
 }
diff --git a/packages/alfa-crawler/test/crawler.spec.ts b/packages/alfa-crawler/test/crawler.spec.ts
index ab35a7df40..04cbb004bf 100644
--- a/packages/alfa-crawler/test/crawler.spec.ts
+++ b/packages/alfa-crawler/test/crawler.spec.ts
@@ -15,7 +15,7 @@ test("#crawl() crawls a frontier", async (t) =>
     for await (const result of crawler.crawl(frontier)) {
       t.equal(result.isOk(), true);
 
-      pages.push(result.get().response.url);
+      pages.push(result.get().response.url.toString());
     }
 
     t.deepEqual(pages, [
diff --git a/packages/alfa-crawler/tsconfig.json b/packages/alfa-crawler/tsconfig.json
index b59557f829..dcea92efe0 100644
--- a/packages/alfa-crawler/tsconfig.json
+++ b/packages/alfa-crawler/tsconfig.json
@@ -21,6 +21,9 @@
     {
       "path": "../alfa-test"
     },
+    {
+      "path": "../alfa-url"
+    },
     {
       "path": "../alfa-web"
     }
diff --git a/packages/alfa-dom/src/style/rule/import.ts b/packages/alfa-dom/src/style/rule/import.ts
index a8ceda76e9..d670ecd6f7 100644
--- a/packages/alfa-dom/src/style/rule/import.ts
+++ b/packages/alfa-dom/src/style/rule/import.ts
@@ -1,4 +1,4 @@
-import { Option } from "@siteimprove/alfa-option";
+import { Option, None } from "@siteimprove/alfa-option";
 import { Trampoline } from "@siteimprove/alfa-trampoline";
 
 import { Rule } from "../rule";
@@ -9,7 +9,7 @@ export class Import extends Condition {
   public static of(
     href: string,
     sheet: Sheet,
-    condition: Option<string>
+    condition: Option<string> = None
   ): Import {
     return new Import(href, sheet, condition);
   }
diff --git a/packages/alfa-frontier/package.json b/packages/alfa-frontier/package.json
index 68b15bc0bb..f79cb66ff0 100644
--- a/packages/alfa-frontier/package.json
+++ b/packages/alfa-frontier/package.json
@@ -21,7 +21,8 @@
     "@siteimprove/alfa-equatable": "^0.5.0",
     "@siteimprove/alfa-json": "^0.5.0",
     "@siteimprove/alfa-option": "^0.5.0",
-    "@siteimprove/alfa-predicate": "^0.5.0"
+    "@siteimprove/alfa-predicate": "^0.5.0",
+    "@siteimprove/alfa-url": "^0.5.0"
   },
   "devDependencies": {
     "@siteimprove/alfa-test": "^0.5.0"
diff --git a/packages/alfa-frontier/src/frontier.ts b/packages/alfa-frontier/src/frontier.ts
index 4fbdc2c0f2..c0d41c8efc 100644
--- a/packages/alfa-frontier/src/frontier.ts
+++ b/packages/alfa-frontier/src/frontier.ts
@@ -2,6 +2,7 @@ import { Equatable } from "@siteimprove/alfa-equatable";
 import { Serializable } from "@siteimprove/alfa-json";
 import { Option, None } from "@siteimprove/alfa-option";
 import { Predicate } from "@siteimprove/alfa-predicate";
+import { URL } from "@siteimprove/alfa-url";
 
 import * as json from "@siteimprove/alfa-json";
 
@@ -35,7 +36,7 @@ export class Frontier implements Equatable, Serializable {
   }
 
   public isInScope(url: string | URL): boolean {
-    return toURL(url).href.startsWith(this._scope.href);
+    return toURL(url).toString().startsWith(this._scope.toString());
   }
 
   public hasWaiting(): boolean {
@@ -186,7 +187,7 @@ export class Frontier implements Equatable, Serializable {
   public equals(value: unknown): value is this {
     return (
       value instanceof Frontier &&
-      value._scope.href === this._scope.href &&
+      value._scope.equals(this._scope) &&
       value._items.length === this._items.length &&
       value._items.every((item, i) => item.equals(this._items[i]))
     );
@@ -194,7 +195,7 @@ export class Frontier implements Equatable, Serializable {
 
   public toJSON(): Frontier.JSON {
     return {
-      scope: this._scope.href,
+      scope: this._scope.toString(),
       items: this._items.map((item) => item.toJSON()),
     };
   }
@@ -269,10 +270,7 @@ class Item implements Equatable, Serializable {
   public matches(url: string | URL): boolean {
     url = toURL(url);
 
-    return (
-      this._url.href === url.href ||
-      this._aliases.some(property("href", equals(url.href)))
-    );
+    return this._url.equals(url) || this._aliases.some(equals(url));
   }
 
   public transition(state: State): boolean {
@@ -304,10 +302,7 @@ class Item implements Equatable, Serializable {
   public alias(url: string | URL): boolean {
     url = toURL(url);
 
-    if (
-      this._url.href === url.href ||
-      this._aliases.some(property("href", equals(url.href)))
-    ) {
+    if (this._url.equals(url) || this._aliases.some(equals(url))) {
       return false;
     }
 
@@ -319,7 +314,7 @@ class Item implements Equatable, Serializable {
   public redirect(target: string | URL): boolean {
     target = toURL(target);
 
-    if (this._url.href === target.href) {
+    if (this._url.equals(target)) {
       return false;
     }
 
@@ -332,19 +327,17 @@ class Item implements Equatable, Serializable {
   public equals(value: unknown): value is this {
     return (
       value instanceof Item &&
-      value._url.href === this._url.href &&
+      value._url.equals(this._url) &&
       value._aliases.length === this._aliases.length &&
-      value._aliases.every(
-        (alias, i) => alias.href === this._aliases[i].href
-      ) &&
+      value._aliases.every((alias, i) => alias.equals(this._aliases[i])) &&
       value._state === this._state
     );
   }
 
   public toJSON(): Item.JSON {
     return {
-      url: this._url.href,
-      aliases: this._aliases.map((url) => url.href),
+      url: this._url.toString(),
+      aliases: this._aliases.map((url) => url.toString()),
       state: this._state,
     };
   }
@@ -360,11 +353,10 @@ namespace Item {
 }
 
 function toURL(url: string | URL): URL {
-  url = typeof url === "string" ? new URL(url) : url;
-  url.hash = "";
-  return url;
+  url = typeof url === "string" ? URL.parse(url).get() : url;
+  return url.withoutFragment();
 }
 
 function isInScope(scope: string | URL, url: string | URL): boolean {
-  return toURL(url).href.startsWith(toURL(scope).href);
+  return toURL(url).toString().startsWith(toURL(scope).toString());
 }
diff --git a/packages/alfa-frontier/test/frontier.spec.ts b/packages/alfa-frontier/test/frontier.spec.ts
index 4d23cd1af9..66b98a91f2 100644
--- a/packages/alfa-frontier/test/frontier.spec.ts
+++ b/packages/alfa-frontier/test/frontier.spec.ts
@@ -106,7 +106,7 @@ test("#enqueue() doesn't change the state of an already seen URL", (t) => {
 test("#dequeue() gets the next waiting URL in queue and moves it to in progress", (t) => {
   const frontier = Frontier.of("https://example.com/");
 
-  t.deepEqual(frontier.dequeue().get(), new URL("https://example.com"));
+  t.deepEqual(frontier.dequeue().get().toString(), "https://example.com/");
 
   t.deepEqual(frontier.toJSON(), {
     scope: "https://example.com/",
diff --git a/packages/alfa-frontier/tsconfig.json b/packages/alfa-frontier/tsconfig.json
index 374857947f..7b813ddeee 100644
--- a/packages/alfa-frontier/tsconfig.json
+++ b/packages/alfa-frontier/tsconfig.json
@@ -17,6 +17,9 @@
     },
     {
       "path": "../alfa-test"
+    },
+    {
+      "path": "../alfa-url"
     }
   ]
 }
diff --git a/packages/alfa-http/package.json b/packages/alfa-http/package.json
index c8943f20bb..c923bb25c2 100644
--- a/packages/alfa-http/package.json
+++ b/packages/alfa-http/package.json
@@ -25,7 +25,8 @@
     "@siteimprove/alfa-json": "^0.5.0",
     "@siteimprove/alfa-map": "^0.5.0",
     "@siteimprove/alfa-option": "^0.5.0",
-    "@siteimprove/alfa-refinement": "^0.5.0"
+    "@siteimprove/alfa-refinement": "^0.5.0",
+    "@siteimprove/alfa-url": "^0.5.0"
   },
   "devDependencies": {
     "@siteimprove/alfa-test": "^0.5.0"
diff --git a/packages/alfa-http/src/request.ts b/packages/alfa-http/src/request.ts
index 7a0549e5ce..a4cc4d2e67 100644
--- a/packages/alfa-http/src/request.ts
+++ b/packages/alfa-http/src/request.ts
@@ -1,4 +1,6 @@
 import { Decoder, Encoder } from "@siteimprove/alfa-encoding";
+import { URL } from "@siteimprove/alfa-url";
+
 import * as earl from "@siteimprove/alfa-earl";
 import * as json from "@siteimprove/alfa-json";
 
@@ -11,25 +13,27 @@ import { Headers } from "./headers";
 export class Request implements Body, json.Serializable, earl.Serializable {
   public static of(
     method: string,
-    url: string,
+    url: URL,
     headers: Headers = Headers.empty(),
     body: ArrayBuffer = new ArrayBuffer(0)
   ): Request {
     return new Request(method, url, headers, body);
   }
 
+  private static _empty = Request.of("GET", URL.parse("about:blank").get());
+
   public static empty(): Request {
-    return Request.of("GET", "about:blank");
+    return this._empty;
   }
 
   private readonly _method: string;
-  private readonly _url: string;
+  private readonly _url: URL;
   private readonly _headers: Headers;
   private readonly _body: ArrayBuffer;
 
   private constructor(
     method: string,
-    url: string,
+    url: URL,
     headers: Headers,
     body: ArrayBuffer
   ) {
@@ -49,7 +53,7 @@ export class Request implements Body, json.Serializable, earl.Serializable {
   /**
    * @see https://fetch.spec.whatwg.org/#dom-request-url
    */
-  public get url(): string {
+  public get url(): URL {
     return this._url;
   }
 
@@ -70,7 +74,7 @@ export class Request implements Body, json.Serializable, earl.Serializable {
   public toJSON(): Request.JSON {
     return {
       method: this._method,
-      url: this._url,
+      url: this._url.toString(),
       headers: this._headers.toJSON(),
       body: Decoder.decode(new Uint8Array(this._body)),
     };
@@ -83,7 +87,7 @@ export class Request implements Body, json.Serializable, earl.Serializable {
       },
       "@type": ["http:Message", "http:Request"],
       "http:methodName": this._method,
-      "http:requestURI": this._url,
+      "http:requestURI": this._url.toString(),
       "http:headers": this._headers.toEARL(),
       "http:body": {
         "@context": {
@@ -130,7 +134,7 @@ export namespace Request {
   export function from(json: JSON): Request {
     return Request.of(
       json.method,
-      json.url,
+      URL.parse(json.url).get(),
       Headers.from(json.headers),
       Encoder.encode(json.body)
     );
diff --git a/packages/alfa-http/src/response.ts b/packages/alfa-http/src/response.ts
index 773a61f28b..ad40dd84c8 100644
--- a/packages/alfa-http/src/response.ts
+++ b/packages/alfa-http/src/response.ts
@@ -1,4 +1,6 @@
 import { Decoder, Encoder } from "@siteimprove/alfa-encoding";
+import { URL } from "@siteimprove/alfa-url";
+
 import * as earl from "@siteimprove/alfa-earl";
 import * as json from "@siteimprove/alfa-json";
 
@@ -10,7 +12,7 @@ import { Headers } from "./headers";
  */
 export class Response implements Body, json.Serializable, earl.Serializable {
   public static of(
-    url: string,
+    url: URL,
     status: number,
     headers: Headers = Headers.empty(),
     body: ArrayBuffer = new ArrayBuffer(0)
@@ -18,17 +20,19 @@ export class Response implements Body, json.Serializable, earl.Serializable {
     return new Response(url, status, headers, body);
   }
 
+  private static _empty = Response.of(URL.parse("about:blank").get(), 200);
+
   public static empty(): Response {
-    return Response.of("about:blank", 200);
+    return this._empty;
   }
 
-  private readonly _url: string;
+  private readonly _url: URL;
   private readonly _status: number;
   private readonly _headers: Headers;
   private readonly _body: ArrayBuffer;
 
   private constructor(
-    url: string,
+    url: URL,
     status: number,
     headers: Headers,
     body: ArrayBuffer
@@ -42,7 +46,7 @@ export class Response implements Body, json.Serializable, earl.Serializable {
   /**
    * @see https://fetch.spec.whatwg.org/#dom-response-url
    */
-  public get url(): string {
+  public get url(): URL {
     return this._url;
   }
 
@@ -69,7 +73,7 @@ export class Response implements Body, json.Serializable, earl.Serializable {
 
   public toJSON(): Response.JSON {
     return {
-      url: this._url,
+      url: this._url.toString(),
       status: this._status,
       headers: this._headers.toJSON(),
       body: Decoder.decode(new Uint8Array(this._body)),
@@ -127,7 +131,7 @@ export namespace Response {
 
   export function from(json: JSON): Response {
     return Response.of(
-      json.url,
+      URL.parse(json.url).get(),
       json.status,
       Headers.from(json.headers),
       Encoder.encode(json.body)
diff --git a/packages/alfa-http/tsconfig.json b/packages/alfa-http/tsconfig.json
index f7f35aa270..ca60786dec 100644
--- a/packages/alfa-http/tsconfig.json
+++ b/packages/alfa-http/tsconfig.json
@@ -38,6 +38,9 @@
     },
     {
       "path": "../alfa-test"
+    },
+    {
+      "path": "../alfa-url"
     }
   ]
 }
diff --git a/packages/alfa-json-ld/package.json b/packages/alfa-json-ld/package.json
index 2123b60ac1..12049c9064 100644
--- a/packages/alfa-json-ld/package.json
+++ b/packages/alfa-json-ld/package.json
@@ -19,7 +19,8 @@
   ],
   "dependencies": {
     "@siteimprove/alfa-option": "^0.5.0",
-    "@siteimprove/alfa-result": "^0.5.0"
+    "@siteimprove/alfa-result": "^0.5.0",
+    "@siteimprove/alfa-url": "^0.5.0"
   },
   "devDependencies": {
     "@siteimprove/alfa-test": "^0.5.0"
diff --git a/packages/alfa-json-ld/src/expand.ts b/packages/alfa-json-ld/src/expand.ts
index 842076cd52..82ce03472d 100644
--- a/packages/alfa-json-ld/src/expand.ts
+++ b/packages/alfa-json-ld/src/expand.ts
@@ -2,6 +2,7 @@
 
 import { None, Option, Some } from "@siteimprove/alfa-option";
 import { Err, Ok, Result } from "@siteimprove/alfa-result";
+import { URL } from "@siteimprove/alfa-url";
 
 import {
   isDictionary,
@@ -871,10 +872,6 @@ function isRelativeIri(url: string): boolean {
   return relativeIri.test(url);
 }
 
-function resolveUrl(target: string, base: string): string {
-  return new URL(target, base).href;
-}
-
 function getMapping(
   context: Context,
   property: string | null,
@@ -957,7 +954,13 @@ function processContext(
         const base = result["@base"];
 
         if (typeof base === "string") {
-          result["@base"] = resolveUrl(value, base);
+          const url = URL.parse(value, base);
+
+          if (url.isErr()) {
+            return url;
+          }
+
+          result["@base"] = url.get().toString();
         }
       }
 
diff --git a/packages/alfa-json-ld/tsconfig.json b/packages/alfa-json-ld/tsconfig.json
index 082638a41d..6d149aa483 100644
--- a/packages/alfa-json-ld/tsconfig.json
+++ b/packages/alfa-json-ld/tsconfig.json
@@ -18,6 +18,9 @@
     },
     {
       "path": "../alfa-test"
+    },
+    {
+      "path": "../alfa-url"
     }
   ]
 }
diff --git a/packages/alfa-scraper/package.json b/packages/alfa-scraper/package.json
index e7adf9e186..33a0893ed2 100644
--- a/packages/alfa-scraper/package.json
+++ b/packages/alfa-scraper/package.json
@@ -26,6 +26,7 @@
     "@siteimprove/alfa-puppeteer": "^0.5.0",
     "@siteimprove/alfa-result": "^0.5.0",
     "@siteimprove/alfa-time": "^0.5.0",
+    "@siteimprove/alfa-url": "^0.5.0",
     "@siteimprove/alfa-web": "^0.5.0",
     "@types/puppeteer": "^3.0.1",
     "puppeteer": "^5.2.1"
diff --git a/packages/alfa-scraper/src/scraper.ts b/packages/alfa-scraper/src/scraper.ts
index 39d2e5983d..7a811fd5c0 100644
--- a/packages/alfa-scraper/src/scraper.ts
+++ b/packages/alfa-scraper/src/scraper.ts
@@ -11,6 +11,7 @@ import { Mapper } from "@siteimprove/alfa-mapper";
 import { Puppeteer } from "@siteimprove/alfa-puppeteer";
 import { Result, Ok, Err } from "@siteimprove/alfa-result";
 import { Timeout } from "@siteimprove/alfa-time";
+import { URL } from "@siteimprove/alfa-url";
 import { Page } from "@siteimprove/alfa-web";
 
 import * as puppeteer from "puppeteer";
@@ -41,10 +42,12 @@ export class Scraper {
     browser?: Promise<puppeteer.Browser>
   ): Promise<T> {
     const scraper = await this.of(browser);
-    const result = await mapper(scraper);
 
-    await scraper.close();
-    return result;
+    try {
+      return await mapper(scraper);
+    } finally {
+      await scraper.close();
+    }
   }
 
   private readonly _browser: puppeteer.Browser;
@@ -60,7 +63,17 @@ export class Scraper {
     url: string | URL,
     options: Scraper.scrape.Options = {}
   ): Promise<Result<Page, string>> {
-    const { href, protocol } = typeof url === "string" ? new URL(url) : url;
+    if (typeof url === "string") {
+      const result = URL.parse(url);
+
+      if (result.isErr()) {
+        return result;
+      }
+
+      url = result.get();
+    }
+
+    const scheme = url.scheme;
 
     const {
       timeout = Timeout.of(10000),
@@ -109,19 +122,19 @@ export class Scraper {
         }, {})
       );
 
-      if (protocol === "http:" || protocol === "https:") {
+      if (scheme === "http" || scheme === "https") {
         await page.setCookie(
           ...[...cookies].map((cookie) => {
             return {
               name: cookie.name,
               value: cookie.value,
-              url: href,
+              url: url.toString(),
             };
           })
         );
       }
 
-      let origin = href;
+      let origin = url.toString();
 
       while (true) {
         try {
@@ -189,7 +202,7 @@ export namespace Scraper {
 function parseRequest(request: puppeteer.Request): Request {
   return Request.of(
     request.method(),
-    request.url(),
+    URL.parse(request.url()).get(),
     Headers.of(
       entries(request.headers()).map(([name, value]) => Header.of(name, value))
     )
@@ -198,7 +211,7 @@ function parseRequest(request: puppeteer.Request): Request {
 
 async function parseResponse(response: puppeteer.Response): Promise<Response> {
   return Response.of(
-    response.url(),
+    URL.parse(response.url()).get(),
     response.status(),
     Headers.of(
       entries(response.headers()).map(([name, value]) => Header.of(name, value))
diff --git a/packages/alfa-scraper/test/scraper.spec.ts b/packages/alfa-scraper/test/scraper.spec.ts
index 7c1177834a..4225295b22 100644
--- a/packages/alfa-scraper/test/scraper.spec.ts
+++ b/packages/alfa-scraper/test/scraper.spec.ts
@@ -11,7 +11,7 @@ test("#scrape() scrapes a page with a hash fragment", async (t) =>
 
     const { response } = result.get();
 
-    t.equal(response.url, url);
+    t.equal(response.url.toString(), url);
   }));
 
 test("#scrape() scrapes a page with an immediate meta refresh", async (t) =>
@@ -23,7 +23,7 @@ test("#scrape() scrapes a page with an immediate meta refresh", async (t) =>
 
     const { response } = result.get();
 
-    t.equal(response.url, "https://example.com/");
+    t.equal(response.url.toString(), "https://example.com/");
   }));
 
 test("#scrape() scrapes a page with a delayed meta refresh", async (t) =>
@@ -35,7 +35,7 @@ test("#scrape() scrapes a page with a delayed meta refresh", async (t) =>
 
     const { response } = result.get();
 
-    t.equal(response.url, url);
+    t.equal(response.url.toString(), url);
   }));
 
 test("#scrape() scrapes a page with an immediate location change", async (t) =>
@@ -47,7 +47,7 @@ test("#scrape() scrapes a page with an immediate location change", async (t) =>
 
     const { response } = result.get();
 
-    t.equal(response.url, "https://example.com/");
+    t.equal(response.url.toString(), "https://example.com/");
   }));
 
 test("#scrape() scrapes a page with a delayed location change", async (t) =>
@@ -59,5 +59,5 @@ test("#scrape() scrapes a page with a delayed location change", async (t) =>
 
     const { response } = result.get();
 
-    t.equal(response.url, url);
+    t.equal(response.url.toString(), url);
   }));
diff --git a/packages/alfa-scraper/tsconfig.json b/packages/alfa-scraper/tsconfig.json
index 1f72b2bb18..a8c1d85ee1 100644
--- a/packages/alfa-scraper/tsconfig.json
+++ b/packages/alfa-scraper/tsconfig.json
@@ -37,6 +37,9 @@
     {
       "path": "../alfa-time"
     },
+    {
+      "path": "../alfa-url"
+    },
     {
       "path": "../alfa-web"
     }
diff --git a/packages/alfa-style/src/property.ts b/packages/alfa-style/src/property.ts
index c49abe3fa7..48ffa94fa4 100644
--- a/packages/alfa-style/src/property.ts
+++ b/packages/alfa-style/src/property.ts
@@ -7,6 +7,9 @@ import * as parser from "@siteimprove/alfa-parser";
 import { Style } from "./style";
 import { Value } from "./value";
 
+/**
+ * @internal
+ */
 export class Property<T = unknown, U = T> {
   public static of<T, U>(
     initial: U,
@@ -53,6 +56,9 @@ export class Property<T = unknown, U = T> {
   }
 }
 
+/**
+ * @internal
+ */
 export namespace Property {
   export interface Options {
     readonly inherits: boolean;
@@ -129,6 +135,9 @@ export namespace Property {
   }
 }
 
+/**
+ * @internal
+ */
 export namespace Property {
   export class Shorthand<N extends Name = never> {
     public static of<N extends Name>(
@@ -222,6 +231,9 @@ const Shorthands = {
   overflow: Overflow.Shorthand,
 };
 
+/**
+ * @internal
+ */
 export namespace Property {
   export type Name = keyof Longhands;
 
@@ -236,6 +248,9 @@ export namespace Property {
   }
 }
 
+/**
+ * @internal
+ */
 export namespace Property {
   export namespace Shorthand {
     export type Name = keyof Shorthands;
diff --git a/packages/alfa-url/package.json b/packages/alfa-url/package.json
new file mode 100644
index 0000000000..aeed6e9963
--- /dev/null
+++ b/packages/alfa-url/package.json
@@ -0,0 +1,36 @@
+{
+  "$schema": "http://json.schemastore.org/package",
+  "name": "@siteimprove/alfa-url",
+  "homepage": "https://siteimprove.com",
+  "version": "0.5.0",
+  "license": "MIT",
+  "description": "Functionality for working with immutable URLs",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/siteimprove/alfa.git",
+    "directory": "packages/alfa-url"
+  },
+  "bugs": "https://github.com/siteimprove/alfa/issues",
+  "main": "src/index.js",
+  "types": "src/index.d.ts",
+  "files": [
+    "src/**/*.js",
+    "src/**/*.d.ts"
+  ],
+  "dependencies": {
+    "@siteimprove/alfa-equatable": "^0.5.0",
+    "@siteimprove/alfa-hash": "^0.5.0",
+    "@siteimprove/alfa-iterable": "^0.5.0",
+    "@siteimprove/alfa-json": "^0.5.0",
+    "@siteimprove/alfa-option": "^0.5.0",
+    "@siteimprove/alfa-result": "^0.5.0",
+    "@siteimprove/alfa-sequence": "^0.5.0"
+  },
+  "devDependencies": {
+    "@siteimprove/alfa-test": "^0.5.0"
+  },
+  "publishConfig": {
+    "access": "public",
+    "registry": "https://npm.pkg.github.com/"
+  }
+}
diff --git a/packages/alfa-url/src/index.ts b/packages/alfa-url/src/index.ts
new file mode 100644
index 0000000000..eaf19f820a
--- /dev/null
+++ b/packages/alfa-url/src/index.ts
@@ -0,0 +1 @@
+export * from "./url";
diff --git a/packages/alfa-url/src/url.ts b/packages/alfa-url/src/url.ts
new file mode 100644
index 0000000000..f97f37e621
--- /dev/null
+++ b/packages/alfa-url/src/url.ts
@@ -0,0 +1,364 @@
+import { Equatable } from "@siteimprove/alfa-equatable";
+import { Hash, Hashable } from "@siteimprove/alfa-hash";
+import { Iterable } from "@siteimprove/alfa-iterable";
+import { Serializable } from "@siteimprove/alfa-json";
+import { Option, None } from "@siteimprove/alfa-option";
+import { Result, Err } from "@siteimprove/alfa-result";
+import { Sequence } from "@siteimprove/alfa-sequence";
+
+import * as json from "@siteimprove/alfa-json";
+
+import { Builtin } from "./url/builtin";
+
+const { isEmpty } = Iterable;
+
+/**
+ * @see https://url.spec.whatwg.org/
+ */
+export class URL implements Equatable, Hashable, Serializable {
+  public static of(
+    scheme: string,
+    username: Option<string> = None,
+    password: Option<string> = None,
+    host: Option<string> = None,
+    port: Option<number> = None,
+    path: Iterable<string> = [],
+    query: Option<string> = None,
+    fragment: Option<string> = None
+  ): URL {
+    return new URL(
+      scheme,
+      username,
+      password,
+      host,
+      port,
+      Sequence.from(path),
+      query,
+      fragment
+    );
+  }
+
+  private readonly _scheme: string;
+  private readonly _username: Option<string>;
+  private readonly _password: Option<string>;
+  private readonly _host: Option<string>;
+  private readonly _port: Option<number>;
+  private readonly _path: Sequence<string>;
+  private readonly _query: Option<string>;
+  private readonly _fragment: Option<string>;
+
+  private constructor(
+    scheme: string,
+    username: Option<string>,
+    password: Option<string>,
+    host: Option<string>,
+    port: Option<number>,
+    path: Sequence<string>,
+    query: Option<string>,
+    fragment: Option<string>
+  ) {
+    this._scheme = scheme;
+    this._username = username;
+    this._password = password;
+    this._host = host;
+    this._port = port;
+    this._path = path;
+    this._query = query;
+    this._fragment = fragment;
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-scheme
+   */
+  public get scheme(): string {
+    return this._scheme;
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-username
+   */
+  public get username(): Option<string> {
+    return this._username;
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-password
+   */
+  public get password(): Option<string> {
+    return this._password;
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-host
+   */
+  public get host(): Option<string> {
+    return this._host;
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-port
+   */
+  public get port(): Option<number> {
+    return this._port;
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-path
+   */
+  public get path(): Sequence<string> {
+    return this._path;
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-query
+   */
+  public get query(): Option<string> {
+    return this._query;
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-fragment
+   */
+  public get fragment(): Option<string> {
+    return this._fragment;
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#include-credentials
+   */
+  public hasCredentials(): boolean {
+    return this._username.isSome() || this._password.isSome();
+  }
+
+  /**
+   * Remove the fragment portion of this URL.
+   *
+   * @remarks
+   * This method is useful for contexts in which the fragment portion of the URL,
+   * which isn't passed from client to server, is of no interest.
+   */
+  public withoutFragment(): URL {
+    if (this._fragment.isNone()) {
+      return this;
+    }
+
+    return new URL(
+      this._scheme,
+      this._username,
+      this._password,
+      this._host,
+      this._port,
+      this._path,
+      this._query,
+      None
+    );
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-equals
+   */
+  public equals(value: URL): boolean;
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-equals
+   */
+  public equals(value: unknown): value is this;
+
+  public equals(value: unknown): boolean {
+    return (
+      value instanceof URL &&
+      value._scheme === this._scheme &&
+      value._username.equals(this._username) &&
+      value._password.equals(this._password) &&
+      value._host.equals(this._host) &&
+      value._port.equals(this._port) &&
+      value._path.equals(this._path) &&
+      value._query.equals(this._query) &&
+      value._fragment.equals(this._fragment)
+    );
+  }
+
+  public hash(hash: Hash): void {
+    Hash.writeString(hash, this._scheme);
+    this._username.hash(hash);
+    this._password.hash(hash);
+    this._host.hash(hash);
+    this._port.hash(hash);
+    this._path.hash(hash);
+    this._query.hash(hash);
+    this._fragment.hash(hash);
+  }
+
+  public toJSON(): URL.JSON {
+    return {
+      scheme: this._scheme,
+      username: this._username.getOr(null),
+      password: this._password.getOr(null),
+      host: this._host.getOr(null),
+      port: this._port.getOr(null),
+      path: this._path.toArray(),
+      query: this._query.getOr(null),
+      fragment: this._fragment.getOr(null),
+    };
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-serializer
+   */
+  public toString(): string {
+    let output = this._scheme + ":";
+
+    for (const host of this._host) {
+      output += "//";
+
+      if (this.hasCredentials()) {
+        for (const username of this._username) {
+          output += username;
+        }
+
+        for (const password of this._password) {
+          output += ":" + password;
+        }
+
+        output += "@";
+      }
+
+      output += host;
+
+      for (const port of this._port) {
+        output += ":" + port.toString(10);
+      }
+    }
+
+    if (this._host.isNone() && this._scheme === "file") {
+      output += "//";
+    }
+
+    if (
+      this._host.isNone() &&
+      this._path.size > 1 &&
+      this._path.first().includes("")
+    ) {
+      output += "/.";
+    }
+
+    for (const segment of this._path) {
+      output += "/" + segment;
+    }
+
+    for (const query of this._query) {
+      output += "?" + query;
+    }
+
+    for (const fragment of this._fragment) {
+      output += "#" + fragment;
+    }
+
+    return output;
+  }
+}
+
+export namespace URL {
+  export interface JSON {
+    [key: string]: json.JSON;
+    scheme: string;
+    username: string | null;
+    password: string | null;
+    host: string | null;
+    port: number | null;
+    path: Array<string>;
+    query: string | null;
+    fragment: string | null;
+  }
+
+  export function from(json: JSON): URL {
+    return URL.of(
+      json.scheme,
+      Option.from(json.username),
+      Option.from(json.password),
+      Option.from(json.host),
+      Option.from(json.port),
+      json.path,
+      Option.from(json.query),
+      Option.from(json.fragment)
+    );
+  }
+
+  /**
+   * @see https://url.spec.whatwg.org/#concept-url-parser
+   *
+   * @remarks
+   * Parsing URLs is tricky business and so this function relies on the presence
+   * of a globally available WHATWG URL class. This API is available in both
+   * browsers, Node.js, and Deno.
+   */
+  export function parse(url: string, base?: string | URL): Result<URL, string> {
+    if (typeof base === "string") {
+      const result = parse(base);
+
+      if (result.isErr()) {
+        return result;
+      }
+
+      base = result.get();
+    }
+
+    try {
+      const {
+        // https://url.spec.whatwg.org/#dom-url-protocol
+        protocol,
+        // https://url.spec.whatwg.org/#dom-url-username
+        username,
+        // https://url.spec.whatwg.org/#dom-url-password
+        password,
+        // https://url.spec.whatwg.org/#dom-url-hostname
+        hostname,
+        // https://url.spec.whatwg.org/#dom-url-port
+        port,
+        // https://url.spec.whatwg.org/#dom-url-pathname
+        pathname,
+        // https://url.spec.whatwg.org/#dom-url-search
+        search,
+        // https://url.spec.whatwg.org/#dom-url-hash
+        hash,
+      } = new Builtin(url, base?.toString());
+
+      return Result.of(
+        URL.of(
+          // `URL#protocol` appends a ":" to the scheme which we need to remove.
+          protocol.replace(/:$/, ""),
+
+          // `URL#username`, `URL#password`, and `URL#hostname` expose the
+          // username, password, and host as-is and so the only thing we need to
+          // do is reject them when empty.
+          Option.of(username).reject(isEmpty),
+          Option.of(password).reject(isEmpty),
+          Option.of(hostname).reject(isEmpty),
+
+          // `URL#port` exposes the port number as a string to we convert it to
+          // a number.
+          Option.of(port).reject(isEmpty).map(Number),
+
+          // `URL#pathname` exposes the path segments with a leading "/" and
+          // joins the segments with "/". We therefore remove the leading "/"
+          // and split the segments by "/" into an array.
+          pathname.replace(/^\//, "").split("/"),
+
+          // `URL#search` exposes the query portion of the URL with a leading
+          // "?" which we need to remove.
+          Option.of(search)
+            .reject(isEmpty)
+            .map((search) => search.replace(/^\?/, "")),
+
+          // `URL#hash` exposes the fragment portion of the URL with a leading
+          // "#" which we need to remove.
+          Option.of(hash)
+            .reject(isEmpty)
+            .map((hash) => hash.replace(/^#/, ""))
+        )
+      );
+    } catch (err) {
+      return Err.of(err.message);
+    }
+  }
+}
diff --git a/packages/alfa-url/src/url/builtin.ts b/packages/alfa-url/src/url/builtin.ts
new file mode 100644
index 0000000000..dcffd5f179
--- /dev/null
+++ b/packages/alfa-url/src/url/builtin.ts
@@ -0,0 +1,4 @@
+/**
+ * @internal
+ */
+export const Builtin = URL;
diff --git a/packages/alfa-url/test/url.spec.ts b/packages/alfa-url/test/url.spec.ts
new file mode 100644
index 0000000000..f726f7dc33
--- /dev/null
+++ b/packages/alfa-url/test/url.spec.ts
@@ -0,0 +1,52 @@
+import { test } from "@siteimprove/alfa-test";
+
+import { URL } from "../src/url";
+
+test(".parse() parses an absolute URL", (t) => {
+  t.deepEqual(URL.parse("https://example.com/page.html").get().toJSON(), {
+    scheme: "https",
+    username: null,
+    password: null,
+    host: "example.com",
+    port: null,
+    path: ["page.html"],
+    query: null,
+    fragment: null,
+  });
+});
+
+test(".parse() parses a relative URL against a base URL", (t) => {
+  t.deepEqual(URL.parse("/page.html", "https://example.com/").get().toJSON(), {
+    scheme: "https",
+    username: null,
+    password: null,
+    host: "example.com",
+    port: null,
+    path: ["page.html"],
+    query: null,
+    fragment: null,
+  });
+});
+
+test(".parse() parses the special about:blank URL", (t) => {
+  t.deepEqual(URL.parse("about:blank").get().toJSON(), {
+    scheme: "about",
+    username: null,
+    password: null,
+    host: null,
+    port: null,
+    path: ["blank"],
+    query: null,
+    fragment: null,
+  });
+});
+
+test("#equals() checks if two URLs are equal", (t) => {
+  const a = URL.parse("foo", "file:").get();
+  const b = URL.parse("foo", "file:").get();
+  const c = URL.parse("bar", "file:").get();
+
+  t.equal(a.equals(a), true);
+  t.equal(a.equals(b), true);
+  t.equal(a.equals(c), false);
+});
diff --git a/packages/alfa-url/tsconfig.json b/packages/alfa-url/tsconfig.json
new file mode 100644
index 0000000000..475adb1076
--- /dev/null
+++ b/packages/alfa-url/tsconfig.json
@@ -0,0 +1,36 @@
+{
+  "$schema": "http://json.schemastore.org/tsconfig",
+  "extends": "../tsconfig.json",
+  "files": [
+    "src/index.ts",
+    "src/url.ts",
+    "src/url/builtin.ts",
+    "test/url.spec.ts"
+  ],
+  "references": [
+    {
+      "path": "../alfa-equatable"
+    },
+    {
+      "path": "../alfa-hash"
+    },
+    {
+      "path": "../alfa-iterable"
+    },
+    {
+      "path": "../alfa-json"
+    },
+    {
+      "path": "../alfa-option"
+    },
+    {
+      "path": "../alfa-result"
+    },
+    {
+      "path": "../alfa-sequence"
+    },
+    {
+      "path": "../alfa-test"
+    }
+  ]
+}
diff --git a/packages/alfa-web/src/page.ts b/packages/alfa-web/src/page.ts
index 4c4450977a..258af9cfff 100644
--- a/packages/alfa-web/src/page.ts
+++ b/packages/alfa-web/src/page.ts
@@ -69,8 +69,8 @@ export class Page implements Resource, json.Serializable, earl.Serializable {
         dct: "http://purl.org/dc/terms/",
       },
       "@type": ["earl:TestSubject"],
-      "@id": this.response.url,
-      "dct:source": this.response.url,
+      "@id": this.response.url.toString(),
+      "dct:source": this.response.url.toString(),
       "dct:hasPart": [this._request.toEARL(), this._response.toEARL()],
     };
   }
diff --git a/packages/tsconfig.json b/packages/tsconfig.json
index 90ef45128a..e6a80e3f75 100644
--- a/packages/tsconfig.json
+++ b/packages/tsconfig.json
@@ -84,6 +84,7 @@
     { "path": "alfa-trampoline" },
     { "path": "alfa-trilean" },
     { "path": "alfa-unexpected" },
+    { "path": "alfa-url" },
     { "path": "alfa-vue" },
     { "path": "alfa-web" },
     { "path": "alfa-webdriver" },