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This is a list of error handling behaviours that might be interesting to define for HTTP header parsing as part of an optional-to-support specification.
Please include references to existing browser behaviours, bugs, etc. where possible.
Multiple Instances of single-value headers
When there are multiple occurrences of a single-value header, what happens? E.g., Content-Length, Location, Host, Content-Type, Date, Location, Referer, Last-Modified, ETag, Range, Content-Range, Age, Expires.
Whitespace around parameter values
The ABNF does not consistently allow whitespace around the '=' in parameters.
Foo: a=b; Foo: c=d;;e=f
These are not legal as per the ABNF, but processors commonly allow them.