In https://github.com/w3c/web-platform-tests/blob/d6102e6097670a68abae60c6c24cc8060d9ac379/fetch/api/cors/cors-filtering.js#L59 there's a test:

corsFilter(url, "Content-Length", "0" , true);

which asks the HTTP server to set the the Content-Length header to be 0. Since the content is top, this causes top to be interpreted as the beginning of the HTTP response to the next HTTP request, so the next test to run on that connection fails. This causes nondeterministic test failure, as seen here: servo/servo#16508 (comment)