An HTTP header field is named information carried in an HTTP message's header section. Its definition determines how its value is interpreted. Field names are case-insensitive, while values follow the rules of their particular field rather than one universal casing rule.[ref]
For example,
ETag: "AbC" and etag: "abc" use the same field name but different entity tags. The quoted opaque strings are compared character by character under the relevant tag-comparison rules. Lowercasing both names is harmless for identifying the field; lowercasing all values would erase a real distinction.[ref]Other fields express different kinds of information.
Content-Type: text/plain; charset=utf-8 describes a media type and character encoding for content. It does not contain the content itself, and its meaning cannot be inferred by applying the entity-tag grammar to it.In HTTP/1.1, a textual field line places a colon between the name and value; an empty line terminates the header section. This is a wire-format example, not a requirement that every HTTP version transmit identical text lines.[ref]
Repeated field occurrences also need their own semantics. Some can be combined as ordered lists, but treating every header section as a dictionary with exactly one value per name can lose information. Read the field definition before combining lines or rearranging values.[ref]
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