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chromium/third_party/blink/web_tests/external/wpt/fetch/h1-parsing/resources/document-with-0x00-in-header.py 

def main(request, response):
    response.headers.set(b"Content-Type", b"text/html")
    response.headers.set(b"Custom", b"\0")
    return b"<!doctype html><script>top.frameLoaded=true</script><b>This is a document.</b>"