diff --git a/src/browser/Mime.zig b/src/browser/Mime.zig
index 5db40a406..16a644fa4 100644
--- a/src/browser/Mime.zig
+++ b/src/browser/Mime.zig
@@ -17,6 +17,7 @@
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 const std = @import("std");
+const Allocator = std.mem.Allocator;
 
 const Mime = @This();
 content_type: ContentType,
@@ -90,29 +91,6 @@ pub fn charsetString(mime: *const Mime) []const u8 {
     return mime.charset[0..mime.charset_len];
 }
 
-/// Removes quotes of value if quotes are given.
-///
-/// Currently we don't validate the charset.
-/// See section 2.3 Naming Requirements:
-/// https://datatracker.ietf.org/doc/rfc2978/
-fn parseCharset(value: []const u8) error{ CharsetTooBig, Invalid }![]const u8 {
-    // Cannot be larger than 40.
-    // https://datatracker.ietf.org/doc/rfc2978/
-    if (value.len > 40) return error.CharsetTooBig;
-
-    // If the first char is a quote, look for a pair.
-    if (value[0] == '"') {
-        if (value.len < 3 or value[value.len - 1] != '"') {
-            return error.Invalid;
-        }
-
-        return value[1 .. value.len - 1];
-    }
-
-    // No quotes.
-    return value;
-}
-
 pub fn parse(input: []const u8) !Mime {
     if (input.len > 255) {
         return error.TooBig;
@@ -128,40 +106,17 @@ pub fn parse(input: []const u8) !Mime {
         return .{ .content_type = content_type };
     }
 
-    const params = trimLeft(normalized[type_len..]);
-
     var charset: [41]u8 = default_charset;
     var charset_len: usize = default_charset_len;
     var has_explicit_charset = false;
 
-    var it = std.mem.splitScalar(u8, params, ';');
-    while (it.next()) |attr| {
-        const i = std.mem.indexOfScalarPos(u8, attr, 0, '=') orelse continue;
-        const name = trimLeft(attr[0..i]);
-
-        const value = trimRight(attr[i + 1 ..]);
-        if (value.len == 0) {
-            continue;
-        }
-
-        const attribute_name = std.meta.stringToEnum(enum {
-            charset,
-        }, name) orelse continue;
-
-        switch (attribute_name) {
-            .charset => {
-                if (value.len == 0) {
-                    break;
-                }
-
-                const attribute_value = parseCharset(value) catch continue;
-                @memcpy(charset[0..attribute_value.len], attribute_value);
-                // Null-terminate right after attribute value.
-                charset[attribute_value.len] = 0;
-                charset_len = attribute_value.len;
-                has_explicit_charset = true;
-            },
-        }
+    // normalized[type_len - 1] is the ';' that terminated the type.
+    var value_buf: [40]u8 = undefined;
+    if (firstCharsetValue(normalized[type_len - 1 ..], &value_buf)) |value| {
+        @memcpy(charset[0..value.len], value);
+        charset[value.len] = 0;
+        charset_len = value.len;
+        has_explicit_charset = true;
     }
 
     mime.charset = charset;
@@ -466,8 +421,298 @@ fn validType(value: []const u8) bool {
     return true;
 }
 
-fn trimLeft(s: []const u8) []const u8 {
-    return std.mem.trimLeft(u8, s, &std.ascii.whitespace);
+/// Parse `input` as a MIME type and return its serialization, or "" on
+/// failure. Unlike `parse` (which classifies a Content-Type for sniffing
+/// and only tracks charset), this preserves every parameter.
+pub fn serialize(arena: Allocator, input: []const u8) ![]const u8 {
+    const trimmed = std.mem.trim(u8, input, &HTTP_WHITESPACE);
+    if (trimmed.len == 0) {
+        return "";
+    }
+
+    // type "/" subtype
+    const slash = std.mem.indexOfScalarPos(u8, trimmed, 0, '/') orelse return "";
+    const type_name = trimmed[0..slash];
+    if (isHttpToken(type_name) == false) {
+        return "";
+    }
+
+    var rest = trimmed[slash + 1 ..];
+    const subtype_end = std.mem.indexOfScalar(u8, rest, ';') orelse rest.len;
+    const subtype = std.mem.trimRight(u8, rest[0..subtype_end], &HTTP_WHITESPACE);
+    if (isHttpToken(subtype) == false) {
+        return "";
+    }
+
+    var out: std.ArrayList(u8) = try .initCapacity(arena, type_name.len + 1 + subtype.len);
+    for (type_name) |c| {
+        out.appendAssumeCapacity(std.ascii.toLower(c));
+    }
+    out.appendAssumeCapacity('/');
+    for (subtype) |c| {
+        out.appendAssumeCapacity(std.ascii.toLower(c));
+    }
+
+    if (subtype_end >= rest.len) {
+        return out.items;
+    }
+
+    rest = rest[subtype_end..]; // positioned at the first ';'
+
+    // The serialized output is the input length plus quoting overhead; reserve
+    // the input length so the common (no-escape) case appends without growing.
+    try out.ensureTotalCapacity(arena, trimmed.len);
+
+    // Lowercased names already emitted, for first-wins dedupe.
+    var seen: std.ArrayList([]const u8) = .empty;
+    // One scratch buffer, reused to unescape quoted values across iterations.
+    var quoted: std.ArrayList(u8) = .empty;
+
+    var i: usize = 0;
+    while (i < rest.len) {
+        i += 1; // skip ';'
+        while (i < rest.len and isHttpWhitespace(rest[i])) i += 1;
+
+        // parameter name: up to ';' or '='
+        const name_start = i;
+        while (i < rest.len and rest[i] != ';' and rest[i] != '=') i += 1;
+        const name = rest[name_start..i];
+
+        if (i >= rest.len) break;
+        if (rest[i] == ';') continue;
+        i += 1; // skip '='
+
+        // A quoted value is unescaped into `quoted`; an unquoted value is used
+        // in place.
+        var value: []const u8 = undefined;
+        if (i < rest.len and rest[i] == '"') {
+            quoted.clearRetainingCapacity();
+            i += 1; // opening quote
+            while (i < rest.len) : (i += 1) {
+                const c = rest[i];
+                if (c == '\\') {
+                    if (i + 1 < rest.len) {
+                        i += 1;
+                        try quoted.append(arena, rest[i]);
+                    } else {
+                        try quoted.append(arena, '\\');
+                    }
+                } else if (c == '"') {
+                    i += 1; // closing quote
+                    break;
+                } else {
+                    try quoted.append(arena, c);
+                }
+            }
+            // ignore any remaining bytes up to the next ';'
+            while (i < rest.len and rest[i] != ';') i += 1;
+            value = quoted.items;
+        } else {
+            const value_start = i;
+            while (i < rest.len and rest[i] != ';') i += 1;
+            value = std.mem.trimRight(u8, rest[value_start..i], &HTTP_WHITESPACE);
+            if (value.len == 0) continue; // empty unquoted value is dropped
+        }
+
+        if (isHttpToken(name) == false) continue;
+        if (isQuotedStringValue(value) == false) continue;
+
+        const lname = try std.ascii.allocLowerString(arena, name);
+        for (seen.items) |s| {
+            if (std.mem.eql(u8, s, lname)) break;
+        } else {
+            try seen.append(arena, lname);
+            try out.append(arena, ';');
+            try out.appendSlice(arena, lname);
+            try out.append(arena, '=');
+            try appendParameterValue(arena, &out, value);
+        }
+    }
+
+    return out.items;
+}
+
+/// HTTP token code points (RFC 7230 §3.2.6). Note these differ from
+/// `VALID_CODEPOINTS` above (which also allows `\`).
+const HTTP_TOKEN = blk: {
+    var v: [256]bool = undefined;
+    for (0..256) |i| {
+        v[i] = switch (i) {
+            '!', '#', '$', '%', '&', '\'', '*', '+', '-', '.', '^', '_', '`', '|', '~' => true,
+            else => std.ascii.isAlphanumeric(@intCast(i)),
+        };
+    }
+    break :blk v;
+};
+
+/// Whether `s` is a non-empty sequence of HTTP token code points. This is also
+/// the definition of a valid header name, so `Headers` reuses it.
+pub fn isHttpToken(s: []const u8) bool {
+    if (s.len == 0) {
+        return false;
+    }
+    for (s) |b| {
+        if (HTTP_TOKEN[b] == false) {
+            return false;
+        }
+    }
+    return true;
+}
+
+/// Whether every code point in `value` is an "HTTP quoted-string token code
+/// point" (HT, 0x20-0x7E, or 0x80-0xFF). Decodes UTF-8 so that a code point
+/// above U+00FF (e.g. U+FFFD) is rejected even though its bytes are each >=
+/// 0x80 — the algorithm operates on code points, not bytes.
+/// https://mimesniff.spec.whatwg.org/#http-quoted-string-token-code-point
+fn isQuotedStringValue(value: []const u8) bool {
+    var i: usize = 0;
+    while (i < value.len) {
+        const n = std.unicode.utf8ByteSequenceLength(value[i]) catch return false;
+        if (i + n > value.len) {
+            return false;
+        }
+        const cp = std.unicode.utf8Decode(value[i..][0..n]) catch return false;
+
+        if (!(cp == 0x09 or (cp >= 0x20 and cp <= 0x7E) or (cp >= 0x80 and cp <= 0xFF))) {
+            return false;
+        }
+        i += n;
+    }
+    return true;
+}
+
+/// HTTP whitespace: HT, LF, CR, SP. Deliberately excludes FF (0x0C) and VT
+/// (0x0B), both of which `std.ascii.whitespace` includes — neither is HTTP
+/// whitespace, so `\x0cx/x` must fail rather than strip to `x/x`.
+pub const HTTP_WHITESPACE = [_]u8{ 0x09, 0x0A, 0x0D, 0x20 };
+
+fn isHttpWhitespace(b: u8) bool {
+    inline for (HTTP_WHITESPACE) |whitespace| {
+        if (b == whitespace) {
+            return true;
+        }
+    }
+    return false;
+}
+
+/// Append a parameter value, quoting (and escaping `"`/`\`) when it is empty
+/// or contains a non-HTTP-token byte.
+fn appendParameterValue(arena: Allocator, out: *std.ArrayList(u8), value: []const u8) !void {
+    if (value.len == 0) {
+        return out.appendSlice(arena, "\"\"");
+    }
+
+    // The clean prefix (up to the first non-token byte) needs no quoting; if
+    // the whole value is tokens, it needs no quotes at all. Neither `"` nor `\`
+    // is a token byte, so the prefix can be copied without escaping.
+    const quote_from = for (value, 0..) |b, i| {
+        if (HTTP_TOKEN[b] == false) break i;
+    } else return out.appendSlice(arena, value);
+
+    // Upper bound: two quotes plus, worst case, a '\' escape for every byte.
+    try out.ensureUnusedCapacity(arena, 2 * value.len + 2);
+    out.appendAssumeCapacity('"');
+    out.appendSliceAssumeCapacity(value[0..quote_from]);
+    for (value[quote_from..]) |b| {
+        if (b == '"' or b == '\\') {
+            out.appendAssumeCapacity('\\');
+        }
+        out.appendAssumeCapacity(b);
+    }
+    out.appendAssumeCapacity('"');
+}
+
+/// Find the value of the first valid `charset` parameter, per the WHATWG MIME
+/// parameter algorithm: parameters are scanned in order, quoted strings honor
+/// `\` escapes and may contain `;`, and the first parameter whose name is
+/// `charset` and whose value is non-empty and contains only HTTP
+/// quoted-string token code points wins. The (unescaped) value is written into
+/// `out`; values longer than `out` are treated as no match. `rest` must begin
+/// at the ';' that ends the subtype, and the input must already be lowercased
+/// (names are matched case-sensitively against "charset").
+fn firstCharsetValue(rest: []const u8, out: []u8) ?[]const u8 {
+    var i: usize = 0;
+    while (i < rest.len) {
+        i += 1; // skip ';'
+        while (i < rest.len and isHttpWhitespace(rest[i])) {
+            i += 1;
+        }
+
+        const name_start = i;
+        while (i < rest.len and rest[i] != ';' and rest[i] != '=') {
+            i += 1;
+        }
+
+        if (i >= rest.len) {
+            break;
+        }
+        if (rest[i] == ';') {
+            continue;
+        }
+
+        const want = std.mem.eql(u8, rest[name_start..i], "charset");
+        i += 1; // skip '='
+
+        var len: usize = 0;
+        var overflow = false;
+        const put = struct {
+            fn put(o: []u8, l: *usize, of: *bool, b: u8) void {
+                if (l.* < o.len) {
+                    o[l.*] = b;
+                    l.* += 1;
+                } else of.* = true;
+            }
+        }.put;
+
+        if (i < rest.len and rest[i] == '"') {
+            i += 1; // opening quote
+            while (i < rest.len) : (i += 1) {
+                const c = rest[i];
+                if (c == '\\') {
+                    if (i + 1 < rest.len) {
+                        i += 1;
+                        if (want) {
+                            put(out, &len, &overflow, rest[i]);
+                        }
+                    } else if (want) {
+                        put(out, &len, &overflow, '\\');
+                    }
+                } else if (c == '"') {
+                    i += 1; // closing quote
+                    break;
+                } else if (want) {
+                    put(out, &len, &overflow, c);
+                }
+            }
+            while (i < rest.len and rest[i] != ';') {
+                i += 1;
+            }
+        } else {
+            const value_start = i;
+            while (i < rest.len and rest[i] != ';') {
+                i += 1;
+            }
+
+            const v = std.mem.trimRight(u8, rest[value_start..i], &HTTP_WHITESPACE);
+            if (v.len == 0) {
+                // empty unquoted value is dropped
+                continue;
+            }
+
+            if (want) {
+                if (v.len <= out.len) {
+                    @memcpy(out[0..v.len], v);
+                    len = v.len;
+                } else overflow = true;
+            }
+        }
+
+        // First *valid* charset wins; otherwise keep scanning.
+        if (want and overflow == false and isQuotedStringValue(out[0..len])) {
+            return out[0..len];
+        }
+    }
+    return null;
 }
 
 fn trimRight(s: []const u8) []const u8 {
@@ -600,6 +845,29 @@ test "Mime: parse charset" {
     }, "text/html;x=\"");
 }
 
+test "Mime: parse charset (WHATWG parameter semantics)" {
+    defer testing.reset();
+
+    // First charset wins (not last).
+    try expect(.{ .content_type = .{ .text_html = {} }, .charset = "gbk" }, "text/html;charset=gbk;charset=utf-8");
+
+    // Backslash escapes inside a quoted value are unescaped.
+    try expect(.{ .content_type = .{ .text_html = {} }, .charset = "gbk" }, "text/html;charset=\"\\g\\b\\k\"");
+
+    // A ';' inside a quoted value is part of the value, not a separator.
+    try expect(.{ .content_type = .{ .text_html = {} }, .charset = "a;b" }, "text/html;charset=\"a;b\";charset=utf-8");
+
+    // A charset whose value isn't all quoted-string tokens (VT here) is
+    // skipped, so the next valid charset wins.
+    try expect(.{ .content_type = .{ .text_html = {} }, .charset = "utf-8" }, "text/html;charset=\x0bgbk;charset=utf-8");
+
+    // A trailing space makes the name "charset " (not "charset"): no charset.
+    try expect(.{ .content_type = .{ .text_html = {} }, .charset = "UTF-8" }, "text/html;charset =gbk");
+
+    // A long preceding parameter doesn't hide a later charset.
+    try expect(.{ .content_type = .{ .text_html = {} }, .charset = "gbk" }, "text/html;" ++ ("a" ** 130) ++ "=x;charset=gbk");
+}
+
 test "Mime: isHTML" {
     defer testing.reset();
 
@@ -724,6 +992,48 @@ fn expect(expected: Expectation, input: []const u8) !void {
     }
 }
 
+test "Mime: serialize" {
+    defer testing.reset();
+    const arena = testing.arena_allocator;
+
+    const expectSerialize = struct {
+        fn call(a: Allocator, input: []const u8, expected: []const u8) !void {
+            try testing.expectString(expected, try Mime.serialize(a, input));
+        }
+    }.call;
+
+    // Essence: lowercased, params preserved verbatim where valid.
+    try expectSerialize(arena, "x/x;bonus=x", "x/x;bonus=x");
+    try expectSerialize(arena, "TEXT/HTML;CHARSET=GBK", "text/html;charset=GBK");
+    try expectSerialize(arena, "text/html;charset=gbk;charset=windows-1255", "text/html;charset=gbk"); // first wins
+    try expectSerialize(arena, "text/html;test;charset=gbk", "text/html;charset=gbk"); // valueless param dropped
+
+    // Quoting on serialize.
+    try expectSerialize(arena, "text/html;charset=gbk(", "text/html;charset=\"gbk(\"");
+    try expectSerialize(arena, "text/html;charset= gbk", "text/html;charset=\" gbk\"");
+    try expectSerialize(arena, "text/html;charset=gbk\"", "text/html;charset=\"gbk\\\"\"");
+    try expectSerialize(arena, "text/html;charset=\"\\g\\b\\k\"", "text/html;charset=gbk"); // unescape
+    try expectSerialize(arena, "text/html;charset=\"\";charset=GBK", "text/html;charset=\"\""); // empty quoted kept
+
+    // Leading/trailing HTTP whitespace (TAB LF CR SP) is stripped; FF/VT are not.
+    try expectSerialize(arena, "\n\r\t x/x;x=x \n\r\t ", "x/x;x=x");
+    try expectSerialize(arena, "\x0cx/x", ""); // FF is not HTTP whitespace -> invalid type
+    try expectSerialize(arena, "x/x\x0c", ""); // FF in subtype -> invalid
+    try expectSerialize(arena, "text/html;\x0ccharset=gbk", "text/html"); // FF before name -> dropped
+
+    // A parameter value containing a code point above U+00FF (here U+FFFD) is
+    // dropped, even though its UTF-8 bytes are each >= 0x80.
+    try expectSerialize(arena, "x/x;test=\u{FFFD};x=x", "x/x;x=x");
+    // 0x80-0xFF round-trips (kept, quoted).
+    try expectSerialize(arena, "x/x;x=\u{00A1};bonus=x", "x/x;x=\"\u{00A1}\";bonus=x");
+
+    // Total failures.
+    try expectSerialize(arena, "", "");
+    try expectSerialize(arena, "x", "");
+    try expectSerialize(arena, "/x", "");
+    try expectSerialize(arena, "x/", "");
+}
+
 test "Mime: prescanCharset" {
     // <meta charset="X">
     try testing.expectEqual("utf-8", Mime.prescanCharset("<html><head><meta charset=\"utf-8\">").?);
diff --git a/src/browser/tests/blob.html b/src/browser/tests/blob.html
index 5f59ade78..dade9b1b7 100644
--- a/src/browser/tests/blob.html
+++ b/src/browser/tests/blob.html
@@ -204,24 +204,25 @@
     testing.expectEqual("application/json", blob.type);
   }
 
-  // MIME with parameters - lowercased
+  // MIME is parsed and re-serialized: optional whitespace dropped, essence
+  // and parameter names lowercased, but parameter values preserved as-is.
   {
     const blob = new Blob([], { type: "text/html; charset=UTF-8" });
-    testing.expectEqual("text/html; charset=utf-8", blob.type);
+    testing.expectEqual("text/html;charset=UTF-8", blob.type);
   }
 
-  // Any ASCII string is accepted and lowercased (no MIME structure validation)
+  // A string that fails to parse as a MIME type yields the empty string.
   {
     const blob = new Blob([], { type: "invalid" });
-    testing.expectEqual("invalid", blob.type);
+    testing.expectEqual("", blob.type);
   }
 
   {
     const blob = new Blob([], { type: "/" });
-    testing.expectEqual("/", blob.type);
+    testing.expectEqual("", blob.type);
   }
 
-  // Non-ASCII characters cause empty string (chars outside U+0020-U+007E)
+  // Non-ASCII characters cause empty string (not HTTP token code points)
   {
     const blob = new Blob([], { type: "ý/x" });
     testing.expectEqual("", blob.type);
@@ -267,7 +268,8 @@
 
     temp = blob.slice(-4, -2, "custom");
     testing.expectEqual(2, temp.size);
-    testing.expectEqual("custom", temp.type);
+    // "custom" is not a valid MIME type (no "/"), so the type is empty.
+    testing.expectEqual("", temp.type);
     testing.async(async () => {
       testing.expectEqual("ai", await temp.text());
     });
diff --git a/src/browser/tests/net/headers.html b/src/browser/tests/net/headers.html
index efa4f48bb..057ec603e 100644
--- a/src/browser/tests/net/headers.html
+++ b/src/browser/tests/net/headers.html
@@ -159,6 +159,36 @@
 }
 </script>
 
+<script id=value-validation>
+// The constructor validates names/values and normalizes values per Fetch.
+{
+  // Leading/trailing HTTP whitespace in values is stripped.
+  const headers = new Headers([["Content-Type", "  text/html\t"]]);
+  testing.expectEqual("text/html", headers.get("content-type"));
+}
+
+// An invalid header name throws a TypeError.
+{
+  let threw = false;
+  try { new Headers([["a b", "x"]]); } catch (e) { threw = e instanceof TypeError; }
+  testing.expectEqual(true, threw);
+}
+
+// A value with a forbidden byte (here an interior newline) throws a TypeError.
+{
+  let threw = false;
+  try { new Headers([["x", "a\nb"]]); } catch (e) { threw = e instanceof TypeError; }
+  testing.expectEqual(true, threw);
+}
+
+// A value with a code point above U+00FF cannot be a byte string: TypeError.
+{
+  let threw = false;
+  try { new Headers({ "x": "Ā" }); } catch (e) { threw = e instanceof TypeError; }
+  testing.expectEqual(true, threw);
+}
+</script>
+
 <script id=copy-constructor>
 // Test creating Headers from another Headers object
 {
diff --git a/src/browser/webapi/Blob.zig b/src/browser/webapi/Blob.zig
index df09e16de..2d4916db7 100644
--- a/src/browser/webapi/Blob.zig
+++ b/src/browser/webapi/Blob.zig
@@ -67,7 +67,7 @@ pub fn init(parts_: ?[]const js.Value, opts_: ?InitOptions, page: *Page) !*Blob
     errdefer session.releaseArena(arena);
 
     const opts: InitOptions = opts_ orelse .{};
-    const mime = try validateMimeType(arena, opts.type, false);
+    const mime = try Mime.serialize(arena, opts.type);
 
     const data = blk: {
         if (parts_) |blob_parts| {
@@ -95,11 +95,11 @@ pub fn init(parts_: ?[]const js.Value, opts_: ?InitOptions, page: *Page) !*Blob
 }
 
 /// Creates a new Blob from raw byte slices (for internal Zig use).
-pub fn initFromBytes(data: []const u8, content_type: []const u8, validate_mime: bool, page: *Page) !*Blob {
+pub fn initFromBytes(data: []const u8, content_type: []const u8, page: *Page) !*Blob {
     const arena = try page.getArena(.large, "Blob");
     errdefer page.releaseArena(arena);
 
-    const mime = try validateMimeType(arena, content_type, validate_mime);
+    const mime = try Mime.serialize(arena, content_type);
 
     const self = try arena.create(Blob);
     self.* = .{
@@ -112,32 +112,6 @@ pub fn initFromBytes(data: []const u8, content_type: []const u8, validate_mime:
     return self;
 }
 
-/// Validates and normalizes MIME type according to spec.
-fn validateMimeType(arena: Allocator, mime_type: []const u8, full_validation: bool) ![]const u8 {
-    if (mime_type.len == 0) {
-        return "";
-    }
-
-    const buf = try arena.dupe(u8, mime_type);
-
-    if (full_validation) {
-        // Full MIME parsing per MIME sniff spec (for Content-Type headers)
-        _ = Mime.parse(buf) catch return "";
-    } else {
-        // Simple validation per FileAPI spec (for Blob constructor):
-        // any char outside U+0020-U+007E yields the empty string.
-        for (mime_type) |c| {
-            if (c < 0x20 or c > 0x7E) {
-                return "";
-            }
-        }
-    }
-
-    // FileAPI: Blob.type is the type converted to ASCII lowercase.
-    _ = std.ascii.lowerString(buf, buf);
-    return buf;
-}
-
 pub fn deinit(self: *Blob, page: *Page) void {
     page.releaseArena(self._arena);
 }
@@ -313,7 +287,7 @@ pub fn slice(
         break :blk @min(data.len, @max(start, @as(u31, @intCast(requested_end))));
     };
 
-    return Blob.initFromBytes(data[start..end], content_type_ orelse "", false, page);
+    return Blob.initFromBytes(data[start..end], content_type_ orelse "", page);
 }
 
 /// Returns the size of the Blob in bytes.
diff --git a/src/browser/webapi/net/FormData.zig b/src/browser/webapi/net/FormData.zig
index 5f8b6986b..1d81db7cc 100644
--- a/src/browser/webapi/net/FormData.zig
+++ b/src/browser/webapi/net/FormData.zig
@@ -597,7 +597,7 @@ test "FormData: multipart empty body" {
 const Blob = @import("../Blob.zig");
 
 fn buildTestFile(arena: Allocator, page: *@import("../../Page.zig"), name: []const u8, mime: []const u8, body: []const u8) !*File {
-    const blob = try Blob.initFromBytes(body, mime, false, page);
+    const blob = try Blob.initFromBytes(body, mime, page);
     blob.acquireRef();
     const file = try arena.create(File);
     file.* = .{
diff --git a/src/browser/webapi/net/Headers.zig b/src/browser/webapi/net/Headers.zig
index f62ea2827..7613f7b09 100644
--- a/src/browser/webapi/net/Headers.zig
+++ b/src/browser/webapi/net/Headers.zig
@@ -2,6 +2,7 @@ const std = @import("std");
 const lp = @import("lightpanda");
 
 const js = @import("../../js/js.zig");
+const Mime = @import("../../Mime.zig");
 
 const KeyValueList = @import("../KeyValueList.zig");
 
@@ -20,11 +21,22 @@ pub const InitOpts = union(enum) {
 };
 
 pub fn init(opts_: ?InitOpts, exec: *const Execution) !*Headers {
-    const list = if (opts_) |opts| switch (opts) {
-        .obj => |obj| try KeyValueList.copy(exec.arena, obj._list),
-        .js_obj => |js_obj| try KeyValueList.fromJsObject(exec.arena, js_obj, normalizeHeaderName, exec.buf),
-        .strings => |kvs| try KeyValueList.fromArray(exec.arena, kvs, normalizeHeaderName, exec.buf),
-    } else KeyValueList.init();
+    const list = blk: {
+        const opts = opts_ orelse break :blk KeyValueList.init();
+        switch (opts) {
+            .obj => |obj| break :blk try KeyValueList.copy(exec.arena, obj._list),
+            .js_obj => |js_obj| {
+                var list = try KeyValueList.fromJsObject(exec.arena, js_obj, normalizeHeaderName, exec.buf);
+                try validateAndNormalize(&list);
+                break :blk list;
+            },
+            .strings => |kvs| {
+                var list = try KeyValueList.fromArray(exec.arena, kvs, normalizeHeaderName, exec.buf);
+                try validateAndNormalize(&list);
+                break :blk list;
+            },
+        }
+    };
 
     return exec._factory.create(Headers{
         ._list = list,
@@ -103,6 +115,42 @@ fn normalizeHeaderName(name: []const u8, buf: []u8) []const u8 {
     return std.ascii.lowerString(buf, name);
 }
 
+/// Validate names and normalize/validate values for a script-provided header
+/// init, trimming values in place. The trim is allocation-free (see
+/// `String.trim`), so an untrimmed value keeps its original storage.
+fn validateAndNormalize(list: *KeyValueList) !void {
+    for (list._entries.items) |*entry| {
+        // A valid header name is exactly a non-empty HTTP token.
+        if (Mime.isHttpToken(entry.name.str()) == false) {
+            return error.TypeError;
+        }
+        const trimmed = entry.value.trim(&Mime.HTTP_WHITESPACE);
+        try validateHeaderValue(trimmed.str());
+        entry.value = trimmed;
+    }
+}
+
+/// Validate an already-normalized header value. Returns `error.TypeError` —
+/// surfaced to script as a JS TypeError — when it contains a code point above
+/// U+00FF (not a valid byte string) or a 0x00/0x0A/0x0D byte.
+/// https://fetch.spec.whatwg.org/#headers-class
+fn validateHeaderValue(value: []const u8) error{TypeError}!void {
+    var i: usize = 0;
+    while (i < value.len) {
+        const n = std.unicode.utf8ByteSequenceLength(value[i]) catch return error.TypeError;
+        if (i + n > value.len) {
+            return error.TypeError;
+        }
+
+        const cp = std.unicode.utf8Decode(value[i..][0..n]) catch return error.TypeError;
+
+        if (cp > 0xFF or cp == 0x00 or cp == 0x0A or cp == 0x0D) {
+            return error.TypeError;
+        }
+        i += n;
+    }
+}
+
 pub const JsApi = struct {
     pub const bridge = js.Bridge(Headers);
 
diff --git a/src/browser/webapi/net/Request.zig b/src/browser/webapi/net/Request.zig
index 1c0854cf2..dccb1b15b 100644
--- a/src/browser/webapi/net/Request.zig
+++ b/src/browser/webapi/net/Request.zig
@@ -223,7 +223,7 @@ pub fn blob(self: *Request, exec: *const Execution) !js.Promise {
     const headers = try self.getHeaders(exec);
     const content_type = try headers.get("content-type", exec) orelse "";
 
-    const b = try Blob.initFromBytes(body, content_type, true, exec.page);
+    const b = try Blob.initFromBytes(body, content_type, exec.page);
     return local.resolvePromise(b);
 }
 
diff --git a/src/browser/webapi/net/Response.zig b/src/browser/webapi/net/Response.zig
index b68149fc5..b33c42aec 100644
--- a/src/browser/webapi/net/Response.zig
+++ b/src/browser/webapi/net/Response.zig
@@ -356,7 +356,7 @@ pub fn blob(self: *Response, exec: *const Execution) !js.Promise {
         .stream => return local.rejectPromise(.{ .type_error = "Cannot read blob from stream body" }),
     };
     const content_type = try self._headers.get("content-type", exec) orelse "";
-    const b = try Blob.initFromBytes(body, content_type, true, exec.page);
+    const b = try Blob.initFromBytes(body, content_type, exec.page);
     return local.resolvePromise(b);
 }
 
diff --git a/src/browser/webapi/net/WebSocket.zig b/src/browser/webapi/net/WebSocket.zig
index 0d59db304..9ce1bf1a9 100644
--- a/src/browser/webapi/net/WebSocket.zig
+++ b/src/browser/webapi/net/WebSocket.zig
@@ -505,7 +505,7 @@ fn dispatchMessageEvent(self: *WebSocket, data: []const u8, frame_type: http.WsF
             switch (self._binary_type) {
                 .arraybuffer => .{ .arraybuffer = .{ .values = data } },
                 .blob => blk: {
-                    const blob = try Blob.initFromBytes(data, "", false, exec.page);
+                    const blob = try Blob.initFromBytes(data, "", exec.page);
                     blob.acquireRef();
                     break :blk .{ .blob = blob };
                 },
diff --git a/src/network/http.zig b/src/network/http.zig
index 3664bac48..663a5b0dc 100644
--- a/src/network/http.zig
+++ b/src/network/http.zig
@@ -248,7 +248,8 @@ pub const AuthChallenge = struct {
 };
 
 pub const ResponseHead = struct {
-    pub const MAX_CONTENT_TYPE_LEN = 64;
+    // Matches Mime.parse's 255-byte cap
+    pub const MAX_CONTENT_TYPE_LEN = 255;
 
     status: u16,
     url: [*c]const u8,
diff --git a/src/string.zig b/src/string.zig
index 26961eb0b..5c93c5c3c 100644
--- a/src/string.zig
+++ b/src/string.zig
@@ -111,6 +111,10 @@ pub const String = packed struct {
         return .init(allocator, self.str(), .{ .dupe = true });
     }
 
+    pub fn trim(self: *const String, values: []const u8) String {
+        return wrap(std.mem.trim(u8, self.str(), values));
+    }
+
     pub fn concat(allocator: Allocator, parts: []const []const u8) !String {
         var total_len: usize = 0;
         for (parts) |part| {
@@ -400,6 +404,23 @@ test "String" {
     }
 }
 
+test "String.trim" {
+    const expect = struct {
+        fn expect(expected: []const u8, input: []const u8, values: []const u8) !void {
+            const s = try String.init(testing.allocator, input, .{});
+            defer s.deinit(testing.allocator);
+            const trimmed = s.trim(values);
+            try testing.expectEqual(expected, trimmed.str());
+        }
+    }.expect;
+
+    try expect("hi", "  hi  ", " "); // SSO, both ends
+    try expect("hello", "hello", " "); // nothing to trim (no allocation)
+    try expect("", "   ", " "); // fully trimmed away
+    try expect("x" ** 20, "   " ++ ("x" ** 20) ++ "\t", &.{ ' ', '\t' }); // heap stays heap (view)
+    try expect("abc", "abc" ++ ("  " ** 6), " "); // heap trims down to SSO
+}
+
 test "String.concat" {
     {
         const result = try String.concat(testing.allocator, &.{});