I worked on standardized parsing for the Content-Type header, such that it is more robust against combining intermediaries (i.e., combine first, then parse). Parsing needed to be standardized as HTTP does not deal with error handling, but browsers do, and were somewhat inconsistent in their handling. https://github.com/whatwg/fetch/pull/831 has the change to Fetch, https://github.com/web-platform-tests/wpt/pull/10525 has tests. (Note that this is different from bug 191356, as that only affects CORS.)
<rdar://problem/48259976>
WPT - https://wpt.fyi/results/fetch/content-type?label=master&label=experimental&aligned=