diff --git a/src/browser/parser/Parser.zig b/src/browser/parser/Parser.zig
index 2cb2acaa..08adfb47 100644
--- a/src/browser/parser/Parser.zig
+++ b/src/browser/parser/Parser.zig
@@ -421,7 +421,16 @@ fn appendBeforeSiblingCallback(ctx: *anyopaque, sibling_ref: *anyopaque, node_or
 fn _appendBeforeSiblingCallback(self: *Parser, sibling: *Node, node_or_text: h5e.NodeOrText) !void {
     const parent = sibling.parentNode() orelse return error.NoParent;
     const node: *Node = switch (node_or_text.toUnion()) {
-        .node => |cpn| getNode(cpn),
+        .node => |cpn| blk: {
+            const child = getNode(cpn);
+            if (child._parent) |previous_parent| {
+                // A custom element constructor may have inserted the node into the
+                // DOM before the parser officially places it (e.g. via foster
+                // parenting). Detach it first so insertNodeRelative's assertion holds.
+                self.page.removeNode(previous_parent, child, .{ .will_be_reconnected = parent.isConnected() });
+            }
+            break :blk child;
+        },
         .text => |txt| try self.page.createTextNode(txt),
     };
     try self.page.insertNodeRelative(parent, node, .{ .before = sibling }, .{});
diff --git a/src/browser/tests/custom_elements/registry.html b/src/browser/tests/custom_elements/registry.html
index 064aa9f9..2d0f5dc0 100644
--- a/src/browser/tests/custom_elements/registry.html
+++ b/src/browser/tests/custom_elements/registry.html
@@ -119,3 +119,33 @@
 }
 </script>
 
+<script id="constructor_self_insert_foster_parent">
+{
+    // Regression: custom element constructor inserting itself (via appendChild) during
+    // innerHTML parsing. When the element is not valid table content, the HTML5 parser
+    // foster-parents it before the <table> via appendBeforeSiblingCallback. That callback
+    // previously didn't check for an existing _parent before calling insertNodeRelative,
+    // causing the "Page.insertNodeRelative parent" assertion to fire.
+    let constructorCalled = 0;
+    let container;
+
+    class CtorSelfInsert extends HTMLElement {
+        constructor() {
+            super();
+            constructorCalled++;
+            // Insert self into container so _parent is set before the parser
+            // officially places this element via appendBeforeSiblingCallback.
+            if (container) container.appendChild(this);
+        }
+    }
+    customElements.define('ctor-self-insert', CtorSelfInsert);
+
+    container = document.createElement('div');
+    // ctor-self-insert is not valid table content; the parser foster-parents it
+    // before the <table>, calling appendBeforeSiblingCallback(sibling=table, node=element).
+    // At that point the element already has _parent=container from the constructor.
+    container.innerHTML = '<table><ctor-self-insert></ctor-self-insert></table>';
+
+    testing.expectEqual(1, constructorCalled);
+}
+</script>
diff --git a/src/http/Client.zig b/src/http/Client.zig
index fde007c6..1a38ef9e 100644
--- a/src/http/Client.zig
+++ b/src/http/Client.zig
@@ -1417,7 +1417,7 @@ pub const Transfer = struct {
             header_len = buf_len - 1;
         }
 
-        const header = buffer[0 .. header_len];
+        const header = buffer[0..header_len];
 
         // We need to parse the first line headers for each request b/c curl's
         // CURLINFO_RESPONSE_CODE returns the status code of the final request.