What version of Bun is running?

Reproduced on both:

• 1.4.3+c6da4a4d3
• 1.4.2+744846f84

What platform is your computer?

Darwin 25.3.0 arm64 arm (uname -mprs)

Browser: Chromium 153.0.8010.12, driven with Playwright.

What steps can reproduce the bug?

Create these four files in an empty directory. No dependencies or framework are needed.

server.ts:

import page from "./index.html";

Bun.serve({
  port: 34143,
  routes: { "/": page },
  development: { hmr: true, console: true },
});

index.html:

<!doctype html>
<html>
  <head>
    <link rel="stylesheet" href="./style.css" />
    <script type="module" src="./client.ts"></script>
  </head>
  <body>
    <button id="reproduce">Add responsive image preload</button>
  </body>
</html>

style.css:

body { font-family: sans-serif; }

client.ts:

document.querySelector("#reproduce")!.addEventListener("click", () => {
  const link = document.createElement("link");
  link.rel = "preload";
  link.as = "image";
  link.imageSrcset = "/small.png 480w, /large.png 800w";
  link.imageSizes = "100vw";
  document.head.appendChild(link);
});

Run bun server.ts, open http://localhost:34143/, and click the button. The image files do not need to exist: the exception is thrown by the CSS HMR observer when the link is inserted, before any image response matters.

I verified this minimal reproduction in a fresh browser context separately under Bun 1.4.2 and 1.4.3. Both produce the same exception and error overlay.

What is the expected behavior?

The CSS HMR observer should ignore non-stylesheet links, including responsive image preloads that use imagesrcset without an href. Adding such a link should not throw or open the runtime error overlay.

What do you see instead?

TypeError: Failed to construct 'URL': Invalid URL
    at EZ (http://localhost:34143/_bun/client/index-00000000059b9630.js:1:785)
    at MutationObserver.<anonymous> (http://localhost:34143/_bun/client/index-00000000059b9630.js:1:585)

The stack above is from 1.4.2. On 1.4.3, the minified function is named Le rather than EZ, at the same :1:785 call site followed by MutationObserver at :1:585. The generated bundle identifier can differ between runs.

Additional information

The issue originally appeared during client-side navigation in a React 19.3.1 application, where responsive image preloads can be inserted into the head. The reproduction above removes React and the framework entirely.

In src/runtime/bake/client/css-reloader.ts, headObserver passes every added HTMLLinkElement to maybeAddCssLink. That function immediately evaluates:

const pathname = new URL(link.href).pathname;

For the preload above, link.href is an empty string, so the URL constructor throws. The initial document scan filters for head > link[rel=stylesheet], but the added-node path does not apply that filter.

A possible fix is to check the link's relation and a nonempty href before parsing the URL. This report does not depend on a particular fix; the regression test should cover dynamically inserted image preloads without an href while CSS HMR is active.