[whatwg] [mimesniff] Review request: Parsing a MIME type

Gordon P. Hemsley gphemsley at gmail.com
Fri May 31 10:30:56 PDT 2013

Hello all,

This is a request seeking feedback and review on the MIME Sniffing
algorithm to "parse a MIME type":


After numerous iterations, I think it is in a state that accurately
reflects the best current practices for interoperability.

As is common with such things, there are numerous points in this
algorithm where implementations do not agree. In general, Firefox and
Chrome tend to pattern together, as do IE and Opera. Safari often
patterns on its own, in favor of a more literal interpretation of the
various RFCs on the matter.

At times, I have had to make a decision as to which was the best
approach. This usually results in half of the implementations being in
violation of the spec; I hope, in those instances, the implementations
in question can be updated to become interoperable with the rest.

With that being said, there are two specific points I want to raise:

(1) The more recent RFCs on the matter restrict type, subtype, and
parameter names to 127 characters. No implementation actually enforces
this limit, but I have included it in the algorithm (relevant points
appear in red) because I think it would be better and safer for both
the user and the user agent to do so.

(2) Based on my analysis of existing implementations, anything that
occurs between the semicolon (and any first whitespace) and the equals
sign is treated as the parameter name, including any whitespace before
the equals sign. However, in order to test parameters, I have been
using 'charset' (because that's they only one I'm aware of that has a
Web-visible effect), and certain implementations may be sniffing
specifically for the string "charset=", which would cloud the results
of my testing. Any enlightenment into this issue would be much

I also have a few general points:

* You may notice in the algorithm that I am using hybrid terminology,
sometimes talking about bytes and sometimes talking about characters.
This is mostly because I haven't decided/determined whether to treat a
MIME type as ASCII or as UTF-8. I think there are arguments on both
sides of the issue, but I'm eager to hear your opinions and advice
(especially about how I might phrase the algorithm if it were written
in terms of characters instead of bytes).

* One of the most controversial parts of this algorithm might be the
issue of what to do when a parameter appears more than once. (The RFCs
suggest that the MIME type should be treated as invalid in such a
case, but no implementation actually treats it that way.) I have opted
to make a later appearance of a parameter override and replace an
earlier appearance of a parameter. Modulo caveat (2) above, this is
only done in half the implementations; in particular, IE and Opera
appear to use the first instance of the parameter as the canonical

* Another important point to notice is the fact that this algorithm
allows parameter names to appear without values. This is useful in
situations such as the "base64" option in data: URLs that use the mere
presence or absence of a parameter to set its boolean value. Note,
however, that a parameter that has been given an explicit value (even
if that value is the empty string) does not get overridden by the
later appearance of a boolean parameter of the same name.

I think those are the important points of background information you
need to know in order to evaluate this algorithm.

I look forward to your response.


Gordon P. Hemsley
me at gphemsley.org

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