diff --git a/packages/alfa-rules/src/sia-r62/rule.ts b/packages/alfa-rules/src/sia-r62/rule.ts
index 7c4ee30e25..12bba61058 100644
--- a/packages/alfa-rules/src/sia-r62/rule.ts
+++ b/packages/alfa-rules/src/sia-r62/rule.ts
@@ -360,11 +360,14 @@ export namespace ComputedStyles {
       "outline-style"
     )} ${longhand("outline-style")}`.trim();
 
-    // While text-decoration-style is not important for deciding if there is one,
-    // it is important for rendering the link with the correct styling.
+    // While text-decoration-style and text-decoration-thickness are not
+    // important for deciding if there is one, but they are important for
+    // rendering the link with the correct styling.
     const textDecoration = `${longhand("text-decoration-line")} ${longhand(
       "text-decoration-color"
-    )} ${longhand("text-decoration-style")}`.trim();
+    )} ${longhand("text-decoration-style")} ${longhand(
+      "text-decoration-thickness"
+    )}`.trim();
 
     const longhands = ([
       "background-color",
diff --git a/packages/alfa-style/src/index.ts b/packages/alfa-style/src/index.ts
index 6071b98438..7e5acd668d 100644
--- a/packages/alfa-style/src/index.ts
+++ b/packages/alfa-style/src/index.ts
@@ -115,6 +115,7 @@ import "./property/text-decoration";
 import "./property/text-decoration-color";
 import "./property/text-decoration-line";
 import "./property/text-decoration-style";
+import "./property/text-decoration-thickness";
 import "./property/text-indent";
 import "./property/text-overflow";
 import "./property/text-transform";
diff --git a/packages/alfa-style/src/property/text-decoration-thickness.ts b/packages/alfa-style/src/property/text-decoration-thickness.ts
new file mode 100644
index 0000000000..03e55f4642
--- /dev/null
+++ b/packages/alfa-style/src/property/text-decoration-thickness.ts
@@ -0,0 +1,64 @@
+import { Keyword, Length, Percentage, Token } from "@siteimprove/alfa-css";
+import { Parser } from "@siteimprove/alfa-parser";
+import { Slice } from "@siteimprove/alfa-slice";
+
+import { Property } from "../property";
+import { Resolver } from "../resolver";
+import { Value } from "../value";
+
+const { either } = Parser;
+
+declare module "../property" {
+  interface Longhands {
+    "text-decoration-thickness": Property<Specified, Computed>;
+  }
+}
+
+/**
+ * @internal
+ */
+export type Specified =
+  | Length
+  | Percentage
+  | Keyword<"auto">
+  | Keyword<"from-font">;
+
+/**
+ * @internal
+ */
+export type Computed = Length<"px"> | Keyword<"auto"> | Keyword<"from-font">;
+
+/**
+ * @internal
+ */
+export const parse = either<Slice<Token>, Specified, string>(
+  Keyword.parse("auto", "from-font"),
+  Length.parse,
+  Percentage.parse
+);
+
+/**
+ * {@link https://developer.mozilla.org/en-US/docs/Web/CSS/outline-width}
+ * @internal
+ */
+export default Property.register(
+  "text-decoration-thickness",
+  Property.of<Specified, Computed>(
+    Keyword.of("auto"),
+    parse,
+    (thickness, style) =>
+      thickness.map((value) => {
+        switch (value.type) {
+          case "keyword":
+            return value;
+          case "length":
+            return Resolver.length(value, style);
+          case "percentage":
+            return Length.of(
+              style.computed("font-size").value.value * value.value,
+              "px"
+            );
+        }
+      })
+  )
+);
diff --git a/packages/alfa-style/src/property/text-decoration.ts b/packages/alfa-style/src/property/text-decoration.ts
index e9f4051ffc..878f492179 100644
--- a/packages/alfa-style/src/property/text-decoration.ts
+++ b/packages/alfa-style/src/property/text-decoration.ts
@@ -6,11 +6,15 @@ import { Property } from "../property";
 import * as Color from "./text-decoration-color";
 import * as Line from "./text-decoration-line";
 import * as Style from "./text-decoration-style";
+import * as Thickness from "./text-decoration-thickness";
 
 declare module "../property" {
   interface Shorthands {
     "text-decoration": Property.Shorthand<
-      "text-decoration-line" | "text-decoration-style" | "text-decoration-color"
+      | "text-decoration-line"
+      | "text-decoration-style"
+      | "text-decoration-color"
+      | "text-decoration-thickness"
     >;
   }
 }
@@ -22,11 +26,17 @@ declare module "../property" {
 export default Property.registerShorthand(
   "text-decoration",
   Property.shorthand(
-    ["text-decoration-line", "text-decoration-style", "text-decoration-color"],
+    [
+      "text-decoration-line",
+      "text-decoration-style",
+      "text-decoration-color",
+      "text-decoration-thickness",
+    ],
     (input) => {
       let line: Line.Specified | undefined;
       let style: Style.Specified | undefined;
       let color: Color.Specified | undefined;
+      let thickness: Thickness.Specified | undefined;
 
       while (true) {
         for (const [remainder] of Token.parseWhitespace(input)) {
@@ -60,11 +70,25 @@ export default Property.registerShorthand(
           }
         }
 
+        if (thickness === undefined) {
+          const result = Thickness.parse(input);
+
+          if (result.isOk()) {
+            [input, thickness] = result.get();
+            continue;
+          }
+        }
+
         break;
       }
 
-      if (line === undefined && style === undefined && color === undefined) {
-        return Err.of(`Expected one of line, style, or color`);
+      if (
+        line === undefined &&
+        style === undefined &&
+        color === undefined &&
+        thickness === undefined
+      ) {
+        return Err.of(`Expected one of line, style, color, or thickness`);
       }
 
       return Result.of([
@@ -73,6 +97,7 @@ export default Property.registerShorthand(
           ["text-decoration-line", line ?? Keyword.of("initial")],
           ["text-decoration-style", style ?? Keyword.of("initial")],
           ["text-decoration-color", color ?? Keyword.of("initial")],
+          ["text-decoration-thickness", thickness ?? Keyword.of("initial")],
         ],
       ]);
     }
diff --git a/packages/alfa-style/test/property/text-decoration.spec.tsx b/packages/alfa-style/test/property/text-decoration.spec.tsx
index dc7e0db954..589bc023e3 100644
--- a/packages/alfa-style/test/property/text-decoration.spec.tsx
+++ b/packages/alfa-style/test/property/text-decoration.spec.tsx
@@ -160,3 +160,54 @@ test("#cascaded() parses `text-decoration: underline solid red`", (t) => {
     source: h.declaration("text-decoration", "underline solid red").toJSON(),
   });
 });
+
+test("#cascaded() parses `text-decoration: underline solid red 2px`", (t) => {
+  const element = <div style={{ textDecoration: "underline solid red 2px" }} />;
+
+  const declaration = h.declaration(
+    "text-decoration",
+    "underline solid red 2px"
+  );
+
+  const style = Style.from(element, device);
+
+  t.deepEqual(style.cascaded("text-decoration-line").get().toJSON(), {
+    value: {
+      type: "list",
+      values: [
+        {
+          type: "keyword",
+          value: "underline",
+        },
+      ],
+      separator: " ",
+    },
+    source: declaration.toJSON(),
+  });
+
+  t.deepEqual(style.cascaded("text-decoration-style").get().toJSON(), {
+    value: {
+      type: "keyword",
+      value: "solid",
+    },
+    source: declaration.toJSON(),
+  });
+
+  t.deepEqual(style.cascaded("text-decoration-color").get().toJSON(), {
+    value: {
+      type: "color",
+      format: "named",
+      color: "red",
+    },
+    source: declaration.toJSON(),
+  });
+
+  t.deepEqual(style.cascaded("text-decoration-thickness").get().toJSON(), {
+    value: {
+      type: "length",
+      value: 2,
+      unit: "px",
+    },
+    source: declaration.toJSON(),
+  });
+});
diff --git a/packages/alfa-style/tsconfig.json b/packages/alfa-style/tsconfig.json
index dda9f4dc68..4644a62e04 100644
--- a/packages/alfa-style/tsconfig.json
+++ b/packages/alfa-style/tsconfig.json
@@ -120,6 +120,7 @@
     "src/property/text-decoration-color.ts",
     "src/property/text-decoration-line.ts",
     "src/property/text-decoration-style.ts",
+    "src/property/text-decoration-thickness.ts",
     "src/property/text-indent.ts",
     "src/property/text-overflow.ts",
     "src/property/text-transform.ts",