diff --git a/src/browser/js/Local.zig b/src/browser/js/Local.zig
index e77c60ac..d45a5e69 100644
--- a/src/browser/js/Local.zig
+++ b/src/browser/js/Local.zig
@@ -120,14 +120,14 @@ pub fn exec(self: *const Local, src: []const u8, name: ?[]const u8) !js.Value {
 /// https://v8.github.io/api/head/classv8_1_1ScriptCompiler.html#a3a15bb5a7dfc3f998e6ac789e6b4646a
 pub fn compileFunction(
     self: *const Local,
-    function_body: []const u8,
+    src: anytype,
     /// We tend to know how many params we'll pass; can remove the comptime if necessary.
     comptime parameter_names: []const []const u8,
     extensions: []const v8.Object,
 ) !js.Function {
     // TODO: Make configurable.
     const script_name = self.isolate.initStringHandle("anonymous");
-    const script_source = self.isolate.initStringHandle(function_body);
+    const script_source = if (@TypeOf(src) == js.String) src.handle else self.isolate.initStringHandle(src);
 
     var parameter_list: [parameter_names.len]*const v8.String = undefined;
     inline for (0..parameter_names.len) |i| {
@@ -742,6 +742,7 @@ fn jsValueToStruct(self: *const Local, comptime T: type, js_val: js.Value) !?T {
                 else => unreachable,
             };
         },
+        js.String => return js_val.isString(),
         string.String => {
             const js_str = js_val.isString() orelse return null;
             return try js_str.toSSO(false);
diff --git a/src/browser/tests/window/timers.html b/src/browser/tests/window/timers.html
index 1b4f4236..5c20dfa5 100644
--- a/src/browser/tests/window/timers.html
+++ b/src/browser/tests/window/timers.html
@@ -51,3 +51,33 @@
   clearImmediate(-3);
   testing.expectEqual(true, true);
 </script>
+
+<script id=setTimeout-string-handler>
+  // Per HTML spec, TimerHandler = Function or DOMString. The string form
+  // is legacy but still required by the spec, and many sites (especially
+  // legacy or server-rendered pages) rely on it.
+  window.__st_string_ran = 0;
+  const st_id = window.setTimeout("window.__st_string_ran = 42;", 1);
+  testing.expectEqual('number', typeof st_id);
+  testing.onload(() => testing.expectEqual(42, window.__st_string_ran));
+</script>
+
+<script id=setInterval-string-handler>
+  window.__si_string_ran = 0;
+  const si_id = window.setInterval("window.__si_string_ran += 1;", 1);
+  testing.expectEqual('number', typeof si_id);
+  window.setTimeout(() => window.clearInterval(si_id), 5);
+  testing.onload(() => testing.expectEqual(true, window.__si_string_ran >= 1));
+</script>
+
+<script id=setTimeout-invalid-handler>
+  // Non-function, non-string handlers must throw a TypeError.
+  let threw = false;
+  try {
+    window.setTimeout(123, 1);
+  } catch (e) {
+    threw = true;
+  }
+  testing.expectEqual(true, threw);
+</script>
+
diff --git a/src/browser/webapi/Window.zig b/src/browser/webapi/Window.zig
index c5673d60..745a94ae 100644
--- a/src/browser/webapi/Window.zig
+++ b/src/browser/webapi/Window.zig
@@ -253,7 +253,13 @@ pub fn fetch(_: *const Window, input: Fetch.Input, options: ?Fetch.InitOpts, pag
     return Fetch.init(input, options, page);
 }
 
-pub fn setTimeout(self: *Window, cb: js.Function.Temp, delay_ms: ?u32, params: []js.Value.Temp, page: *Page) !u32 {
+const LegacyHandler = union(enum) {
+    function: js.Function.Temp,
+    string: js.String,
+};
+
+pub fn setTimeout(self: *Window, handler: LegacyHandler, delay_ms: ?u32, params: []js.Value.Temp, page: *Page) !u32 {
+    const cb = try resolveTimerHandler(handler, page);
     return self.scheduleCallback(cb, delay_ms orelse 0, .{
         .repeat = false,
         .params = params,
@@ -262,7 +268,8 @@ pub fn setTimeout(self: *Window, cb: js.Function.Temp, delay_ms: ?u32, params: [
     }, page);
 }
 
-pub fn setInterval(self: *Window, cb: js.Function.Temp, delay_ms: ?u32, params: []js.Value.Temp, page: *Page) !u32 {
+pub fn setInterval(self: *Window, handler: LegacyHandler, delay_ms: ?u32, params: []js.Value.Temp, page: *Page) !u32 {
+    const cb = try resolveTimerHandler(handler, page);
     return self.scheduleCallback(cb, delay_ms orelse 0, .{
         .repeat = true,
         .params = params,
@@ -271,6 +278,21 @@ pub fn setInterval(self: *Window, cb: js.Function.Temp, delay_ms: ?u32, params:
     }, page);
 }
 
+// https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#dom-settimeout
+// https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#timerhandler
+// TimerHandler = Function or DOMString. When a string is passed, it is
+// compiled into an anonymous function body, matching how legacy browsers
+// (and all current UAs) interpret `setTimeout("foo()", 100)`.
+fn resolveTimerHandler(handler: LegacyHandler, page: *Page) !js.Function.Temp {
+    switch (handler) {
+        .function => |fun| return fun,
+        .string => |str| {
+            const fun = try page.js.local.?.compileFunction(str, &.{}, &.{});
+            return fun.temp();
+        },
+    }
+}
+
 pub fn setImmediate(self: *Window, cb: js.Function.Temp, params: []js.Value.Temp, page: *Page) !u32 {
     return self.scheduleCallback(cb, 0, .{
         .repeat = false,