DOCTYPE header missing in https://widgets.wp.com/3rd-party-cookie-check/complete.html

### Impacted plugin

Jetpack

### Steps to Reproduce

1. Go to https://widgets.wp.com/3rd-party-cookie-check/complete.html
2. Open Developer Tools in browser
3. Check Debugger for 3rd-party-cookie-check > complete.html
4. An error shows `This page is in Quirks Mode. Page layout may be impacted. For Standards Mode use “”.`
5. Links to here: https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode

### A clear and concise description of what you expected to happen.

The document complete.html should begin with ``

### What actually happened

It does not:

```

if (window.parent) {
if (/wpcom-thirdparty-cookie-check=ok/.test(document.cookie)) {
window.parent.postMessage('WPCOM:3PC:allowed', '*');
} else {
window.parent.postMessage('WPCOM:3PC:blocked', '*');
}
document.cookie = 'wpcom-thirdparty-cookie-check=; expires=Thu, 01 Jan 1970 00:00:01 GMT;';
}

```

### Browser

Mozilla Firefox

### Other information


### Platform (Simple, Atomic, or both?)

_No response_

### Reproducibility

Consistent

### Severity

_No response_

### Available workarounds?

_No response_

### Workaround details

_No response_

Start with the complete.html document served at https://widgets.wp.com/3rd-party-cookie-check/complete.html and compare its opening markup with the issue’s example. Done means the document begins with the requested HTML doctype and the page no longer reports Quirks Mode in browser Developer Tools.

Written by the indexing model from the issue text.