And yet HTTP/1.1 headers should be sent in Latin1 (is this fixed in HTTP/2 or HTTP/3?). And WebKit's JavaScriptCore has special handling for Latin1 strings in JS, for performance reasons I assume.
Do you have a source on that "should" part. Because the spec disagrees https://www.rfc-editor.org/rfc/rfc7230#section-3.2.4:
> Historically, HTTP has allowed field content with text in the ISO-8859-1 charset [ISO-8859-1], supporting other charsets only through use of [RFC2047] encoding. In practice, most HTTP header field values use only a subset of the US-ASCII charset [USASCII]. Newly defined header fields SHOULD limit their field values to US-ASCII octets.
In practice and by spec, HTTP headers should be ASCII encoded.
The section you quoted actually suggests that implementations should support ISO-8859-1 to ensure compatibility with systems that use it.
> Newly defined header fields SHOULD limit their field values to US-ASCII octets
ASCII octets! That means you SHOULD NOT send Latin1 encoded headers. The opposite of what pzmarzly was saying. I don't disagree Latin-1 being a superset of ASCII or having backward compatibility in mind, but that's not relevant to my response.
> SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
Web servers need to be able to receive and decode latin1 into utf-8 regardless of what the RFC recommends people send. The fact that it's going to become rarer over time to have the 8th bit set in headers, means you can write a simpler algorithm than what Lemire did that assumes an ASCII average case. https://github.com/jart/cosmopolitan/blob/755ae64e73ef5ef7d1... That goes 23 GB/s on my machine using just SSE2 (rather than AVX512). However it goes much slower if the text is full of european diacritics. Lemire's algorithm is better at decoding those.
Otherwise known as "Making other people's incompetence and inability to implement a specification your problem." Just because it's a widely quoted maxim doesn't make it good advice.
I do agree that latin1 headers shouldn't be _sent_ out though.