diff --git a/src/browser/tests/element/css_style_properties.html b/src/browser/tests/element/css_style_properties.html
index e2ce4e3b8..3100d2cd2 100644
--- a/src/browser/tests/element/css_style_properties.html
+++ b/src/browser/tests/element/css_style_properties.html
@@ -60,6 +60,16 @@
 
   div.style.setProperty('float', 'right');
   testing.expectEqual('right', div.style.cssFloat);
+
+  // undefined stringifies to "undefined", it doesn't clear the declaration
+  // (Chrome rejects the invalid value and keeps 'right'; we don't validate yet)
+  div.style.cssFloat = undefined;
+  testing.expectEqual(false, div.style.cssFloat === '');
+
+  // [LegacyNullToEmptyString]: null removes it
+  div.style.cssFloat = null;
+  testing.expectEqual('', div.style.cssFloat);
+  testing.expectEqual('', div.style.cssText);
 }
 </script>
 
diff --git a/src/browser/webapi/css/CSSStyleDeclaration.zig b/src/browser/webapi/css/CSSStyleDeclaration.zig
index 0d66df276..b23142e01 100644
--- a/src/browser/webapi/css/CSSStyleDeclaration.zig
+++ b/src/browser/webapi/css/CSSStyleDeclaration.zig
@@ -485,11 +485,13 @@ fn getFloat(self: *const CSSStyleDeclaration, frame: *Frame) []const u8 {
     return self.getPropertyValue("float", frame);
 }
 
-fn setFloat(self: *CSSStyleDeclaration, value_: ?[]const u8, frame: *Frame) !void {
+fn setFloat(self: *CSSStyleDeclaration, value: js.Value, frame: *Frame) !void {
     if (self._is_computed) {
         return error.NoModificationAllowed;
     }
-    if (try self.setPropertyImpl("float", value_ orelse "", false, frame)) {
+    // `[LegacyNullToEmptyString] CSSOMString`: null becomes "", undefined becomes "undefined".
+    const str = if (value.isNull()) "" else try value.toZig([]const u8);
+    if (try self.setPropertyImpl("float", str, false, frame)) {
         try self.syncStyleAttribute(frame);
     }
 }