diff --git a/media_types/_util.ts b/media_types/_util.ts
index 50d3282991bd..360809c96eec 100644
--- a/media_types/_util.ts
+++ b/media_types/_util.ts
@@ -131,7 +131,7 @@ function isTokenChar(r: string): boolean {
   return code > 0x20 && code < 0x7f && !isTSpecial(r);
 }
 
-function isTSpecial(r: string): boolean {
+export function isTSpecial(r: string): boolean {
   return r[0] ? `()<>@,;:\\"/[]?=`.includes(r[0]) : false;
 }
 
diff --git a/media_types/_util_test.ts b/media_types/_util_test.ts
index afcf81cf1633..4dc34b8b7685 100644
--- a/media_types/_util_test.ts
+++ b/media_types/_util_test.ts
@@ -1,7 +1,15 @@
 // Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
 
 import { assertEquals } from "../assert/mod.ts";
-import { consumeMediaParam, consumeToken, consumeValue } from "./_util.ts";
+import {
+  consumeMediaParam,
+  consumeToken,
+  consumeValue,
+  decode2331Encoding,
+  isIterator,
+  isToken,
+  isTSpecial,
+} from "./_util.ts";
 
 Deno.test({
   name: "consumeToken()",
@@ -22,6 +30,10 @@ Deno.test({
   name: "consumeValue()",
   fn() {
     const fixtures = [
+      ["", "", ""],
+      [`"\n"foo`, "", `"\n"foo`],
+      [`"\r"foo`, "", `"\r"foo`],
+      [`"\\\\;"`, "\\;", ""],
       ["foo bar", "foo", " bar"],
       ["bar", "bar", ""],
       [" bar ", "", " bar "],
@@ -47,6 +59,10 @@ Deno.test({
   name: "consumeMediaParam()",
   fn() {
     const fixtures = [
+      ["", "", "", ""],
+      ["foo=bar", "", "", "foo=bar"],
+      [";", "", "", ";"],
+      [";foo", "", "", ";foo"],
       [" ; foo=bar", "foo", "bar", ""],
       ["; foo=bar", "foo", "bar", ""],
       [";foo=bar", "foo", "bar", ""],
@@ -69,3 +85,87 @@ Deno.test({
     }
   },
 });
+
+Deno.test({
+  name: "decode2331Encoding()",
+  fn() {
+    const fixtures = [
+      ["", undefined],
+      ["foo", undefined],
+      [`foo'bar'baz`, undefined],
+      [`us-ascii'en-us'`, undefined],
+      [`us-ascii'en-us'This%20is%20%2A%2A%2Afun%2A%2A%2A`, "This is ***fun***"],
+      [`UTF-8''foo-a%cc%88.html`, "foo-ä.html"],
+    ] as const;
+    for (const [fixture, expected] of fixtures) {
+      assertEquals(decode2331Encoding(fixture), expected);
+    }
+  },
+});
+
+Deno.test({
+  name: "isIterator()",
+  fn() {
+    const fixtures = [
+      [null, false],
+      [undefined, false],
+      [{}, false],
+      ["", true],
+      [[], true],
+      [Object.entries({}), true],
+    ] as const;
+    for (const [fixture, expected] of fixtures) {
+      assertEquals(isIterator(fixture), expected);
+    }
+  },
+});
+
+Deno.test({
+  name: "isToken()",
+  fn() {
+    const fixtures = [
+      ["", false],
+      [";", false],
+      ["\\", false],
+      ["foo", true],
+    ] as const;
+    for (const [fixture, expected] of fixtures) {
+      assertEquals(isToken(fixture), expected);
+    }
+  },
+});
+
+Deno.test({
+  name: "isTSpecial()",
+  fn() {
+    const fixtues = [
+      ["", false],
+      [` ()<>@,;:\\"/[]?=`, false],
+      ["(", true],
+      [")", true],
+      ["<", true],
+      [">", true],
+      ["@", true],
+      [",", true],
+      [";", true],
+      [":", true],
+      ["\\", true],
+      ['"', true],
+      ["/", true],
+      ["[", true],
+      ["]", true],
+      ["?", true],
+      ["=", true],
+      [" ", false],
+      ["\t", false],
+      ["\n", false],
+      ["\r", false],
+      ["\f", false],
+      ["\v", false],
+      ["foo", false],
+    ] as const;
+    for (const [fixture, expected] of fixtues) {
+      assertEquals(isTSpecial(fixture), expected);
+    }
+  },
+});
diff --git a/media_types/content_type_test.ts b/media_types/content_type_test.ts
index 8eeaed408f35..51df27da4006 100644
--- a/media_types/content_type_test.ts
+++ b/media_types/content_type_test.ts
@@ -7,10 +7,12 @@ Deno.test({
   name: "contentType()",
   fn() {
     const fixtures = [
+      [" ; charset=UTF-8", undefined],
       [".json", "application/json; charset=UTF-8"],
       ["text/html", "text/html; charset=UTF-8"],
       ["txt", "text/plain; charset=UTF-8"],
       ["text/plain; charset=ISO-8859-1", "text/plain; charset=ISO-8859-1"],
+      ["text/plan; charset", undefined],
       ["foo", undefined],
       ["file.json", undefined],
       ["application/foo", "application/foo"],
diff --git a/media_types/extensions_by_type_test.ts b/media_types/extensions_by_type_test.ts
index 7306ecbd75bb..f5dede5e19de 100644
--- a/media_types/extensions_by_type_test.ts
+++ b/media_types/extensions_by_type_test.ts
@@ -7,6 +7,7 @@ Deno.test({
   name: "extensionsByType()",
   fn() {
     const fixtures: [string, string[] | undefined][] = [
+      ["text/plain; charset", undefined],
       ["image/gif", ["gif"]],
       ["application/javascript", ["js", "mjs"]],
       ["text/html; charset=UTF-8", ["html", "htm", "shtml"]],
diff --git a/media_types/format_media_type_test.ts b/media_types/format_media_type_test.ts
index 4ceb97322f25..bdcfcba27984 100644
--- a/media_types/format_media_type_test.ts
+++ b/media_types/format_media_type_test.ts
@@ -7,6 +7,7 @@ Deno.test({
   name: "formatMediaType()",
   fn() {
     const fixtures = [
+      ["/", undefined, ""],
       ["noslash", { X: "Y" }, "noslash; x=Y"],
       ["foo bar/baz", undefined, ""],
       ["foo/bar baz", undefined, ""],
@@ -58,6 +59,7 @@ Deno.test({
       ],
       ["foo/bar", { "0": "'", "9": "'" }, "foo/bar; 0='; 9='"],
       ["foo", { "bar": "" }, `foo; bar=""`],
+      ["foo/bar", [], "foo/bar"],
     ] as const;
     for (const [type, param, expected] of fixtures) {
       assertEquals(formatMediaType(type, param), expected);
diff --git a/media_types/get_charset_test.ts b/media_types/get_charset_test.ts
index 466ea7f3be23..ba8d78a9a877 100644
--- a/media_types/get_charset_test.ts
+++ b/media_types/get_charset_test.ts
@@ -7,6 +7,8 @@ Deno.test({
   name: "getCharset()",
   fn() {
     const fixtures = [
+      [";", undefined],
+      ["text/plain; charset", undefined],
       ["text/plain", "UTF-8"],
       ["text/html", "UTF-8"],
       ["application/foo", undefined],
diff --git a/media_types/parse_media_type_test.ts b/media_types/parse_media_type_test.ts
index 5195578149d8..d76748aec23c 100644
--- a/media_types/parse_media_type_test.ts
+++ b/media_types/parse_media_type_test.ts
@@ -1,6 +1,6 @@
 // Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
 
-import { assertEquals } from "../assert/mod.ts";
+import { assertEquals, assertThrows } from "../assert/mod.ts";
 import { parseMediaType } from "./mod.ts";
 
 Deno.test({
@@ -91,3 +91,40 @@ Deno.test({
     }
   },
 });
+
+Deno.test({
+  name: "parseMediaType() throws on invalid media type",
+  fn() {
+    const fixtures = [
+      `form-data; foo`,
+      `form-data; foo="bar"; baz`,
+    ] as const;
+    for (const fixture of fixtures) {
+      assertThrows(
+        () => {
+          parseMediaType(fixture);
+        },
+        TypeError,
+        "Invalid media parameter.",
+      );
+    }
+  },
+});
+
+Deno.test({
+  name: "parseMediaType() throws on duplicate keys",
+  fn() {
+    const fixtures = [
+      `form-data; foo="bar"; foo="baz"`,
+    ] as const;
+    for (const fixture of fixtures) {
+      assertThrows(
+        () => {
+          parseMediaType(fixture);
+        },
+        TypeError,
+        "Duplicate key parsed.",
+      );
+    }
+  },
+});