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Remove sniffing of HTML
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As no user agent today appears to identify a text/html resource starting with <rss as XML, remove those rules from the standard.

At the same time, make it more clear that XML (and now HTML) are never sniffed. This part is a non-normative change for clarity.

Tests: web-platform-tests/wpt#47002.

Closes #173.
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@annevk
annevk authored Jul 15, 2024
1 parent 9fd7a60 commit b47be44
Showing 1 changed file with 6 additions and 287 deletions.
293 changes: 6 additions & 287 deletions mimesniff.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -1806,6 +1806,12 @@ algorithm</dfn>:
user agents must use the following <dfn>MIME type sniffing algorithm</dfn>:

<ol>
<li>
If the <a>supplied MIME type</a> is an <a>XML MIME type</a> or <a>HTML MIME type</a>, the
<a>computed MIME type</a> is the <a>supplied MIME type</a>.

Abort these steps.

<li>
If the <a>supplied MIME type</a> is undefined or if the
<a>supplied MIME type</a>'s <a for="MIME type">essence</a> is
Expand All @@ -1826,17 +1832,6 @@ algorithm</dfn>:
<a>rules for distinguishing if a resource is text or binary</a> and
abort these steps.

<li>
If the <a>supplied MIME type</a> is an <a>XML MIME type</a>, the
<a>computed MIME type</a> is the <a>supplied MIME type</a>.

Abort these steps.

<li>
If the <a>supplied MIME type</a>'s <a for="MIME type">essence</a> is "<code>text/html</code>",
execute the <a>rules for distinguishing if a resource is a feed or HTML</a> and
abort these steps.

<li>
If the <a>supplied MIME type</a> is an <a>image MIME type</a>
<a>supported by the user agent</a>, let <var>matched-type</var> be
Expand Down Expand Up @@ -2264,9 +2259,6 @@ type</dfn>:

</table>

<p class=XXX>
What about feeds?

<li>
<p>Execute the following steps for each row <var>row</var> in the following table:

Expand Down Expand Up @@ -2466,279 +2458,6 @@ type</dfn>:



<h3 id=sniffing-a-mislabeled-feed>Sniffing a mislabeled feed</h3>

<p>
To determine whether a feed has been mislabeled as HTML, execute the
following <dfn>rules for distinguishing if a resource is a feed or
HTML</dfn>:

<ol>
<li>
Let <var>sequence</var> be the <a>resource header</a>, where
<var>sequence</var>[<var>s</var>] is <a>byte</a> <var>s</var> in
<var>sequence</var> and <var>sequence</var>[0] is the first
<a>byte</a> in <var>sequence</var>.

<li>
Let <var>length</var> be the number of <a>bytes</a> in
<var>sequence</var>.

<li>
Initialize <var>s</var> to 0.

<li>
If <var>length</var> is greater than or equal to 3 and the three
<a>bytes</a> from <var>sequence</var>[0] to
<var>sequence</var>[2] are equal to 0xEF 0xBB 0xBF (UTF-8 BOM), increment
<var>s</var> by 3.

<li>
While <var>s</var> is less than <var>length</var>, continuously loop
through these steps:

<ol>
<li>
Enter loop <var>L</var>:

<ol>
<li>
If <var>sequence</var>[<var>s</var>] is undefined, the <a>computed
MIME type</a> is the <a>supplied MIME type</a>.

Abort these steps.

<li>
If <var>sequence</var>[<var>s</var>] is equal to 0x3C
("<code>&lt;</code>"), increment <var>s</var> by 1 and exit loop
<var>L</var>.

<li>
If <var>sequence</var>[<var>s</var>] is not a <a>whitespace
byte</a>, the <a>computed MIME type</a> is the <a>supplied
MIME type</a>.

Abort these steps.

<li>
Increment <var>s</var> by 1.
</ol>

<li>
Enter loop <var>L</var>:

<ol>
<li>
If <var>sequence</var>[<var>s</var>] is undefined, the <a>computed
MIME type</a> is the <a>supplied MIME type</a>.

Abort these steps.

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 3 and
the three <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 2] are equal to 0x21 0x2D 0x2D
("<code>!--</code>"), increment <var>s</var> by 3 and enter loop
<var>M</var>:

<ol>
<li>
If <var>sequence</var>[<var>s</var>] is undefined, the <a>computed
MIME type</a> is the <a>supplied MIME type</a>.

Abort these steps.

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 3 and
the three <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 2] are equal to 0x2D 0x2D 0x3E
("<code>--></code>"), increment <var>s</var> by 3 and exit
loops <var>M</var> and <var>L</var>.

<li>
Increment <var>s</var> by 1.
</ol>

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 1 and
<var>sequence</var>[<var>s</var>] is equal to 0x21
("<code>!</code>"), increment <var>s</var> by 1 and enter loop
<var>M</var>:

<ol>
<li>
If <var>sequence</var>[<var>s</var>] is undefined, the <a>computed
MIME type</a> is the <a>supplied MIME type</a>.

Abort these steps.

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 1 and
<var>sequence</var>[<var>s</var>] is equal to 0x3E
("<code>></code>"), increment <var>s</var> by 1 and exit loops
<var>M</var> and <var>L</var>.

<li>
Increment <var>s</var> by 1.
</ol>

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 1 and
<var>sequence</var>[<var>s</var>] is equal to 0x3F
("<code>?</code>"), increment <var>s</var> by 1 and enter loop
<var>M</var>:

<ol>
<li>
If <var>sequence</var>[<var>s</var>] is undefined, the <a>computed
MIME type</a> is the <a>supplied MIME type</a>.

Abort these steps.

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 2 and
the two <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 1] are equal to 0x3F 0x3E
("<code>?></code>"), increment <var>s</var> by 2 and exit loops
<var>M</var> and <var>L</var>.

<li>
Increment <var>s</var> by 1.
</ol>

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 3 and
the three <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 2] are equal to 0x72 0x73 0x73
("<code>rss</code>"), the <a>computed MIME type</a> is
"<code>application/rss+xml</code>".

Abort these steps.

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 4 and
the four <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 3] are equal to 0x66 0x65 0x65 0x64
("<code>feed</code>"), the <a>computed MIME type</a> is
"<code>application/atom+xml</code>".

Abort these steps.

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 7 and
the seven <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 6] are equal to 0x72 0x64 0x66 0x3A
0x52 0x44 0x46 ("<code>rdf:RDF</code>"), increment <var>s</var>
by 7 and enter loop <var>M</var>:

<ol>
<li>
If <var>sequence</var>[<var>s</var>] is undefined, the <a>computed
MIME type</a> is the <a>supplied MIME type</a>.

Abort these steps.

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 24
and the twenty-four <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 23] are equal to 0x68 0x74 0x74
0x70 0x3A 0x2F 0x2F 0x70 0x75 0x72 0x6C 0x2E 0x6F 0x72 0x67 0x2F 0x72
0x73 0x73 0x2F 0x31 0x2E 0x30 0x2F
("<code>http://purl.org/rss/1.0/</code>"), increment
<var>s</var> by 24 and enter loop <var>N</var>:

<ol>
<li>
If <var>sequence</var>[<var>s</var>] is undefined, the
<a>computed MIME type</a> is the <a>supplied MIME
type</a>.

Abort these steps.

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 43
and the forty-three <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 42] are equal to 0x68 0x74 0x74
0x70 0x3A 0x2F 0x2F 0x77 0x77 0x77 0x2E 0x77 0x33 0x2E 0x6F 0x72
0x67 0x2F 0x31 0x39 0x39 0x39 0x2F 0x30 0x32 0x2F 0x32 0x32 0x2D
0x72 0x64 0x66 0x2D 0x73 0x79 0x6E 0x74 0x61 0x78 0x2D 0x6E 0x73
0x23
("<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#</code>"),
the <a>computed MIME type</a> is
"<code>application/rss+xml</code>".

Abort these steps.

<li>
Increment <var>s</var> by 1.
</ol>

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 24
and the twenty-four <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 23] are equal to 0x68 0x74 0x74
0x70 0x3A 0x2F 0x2F 0x77 0x77 0x77 0x2E 0x77 0x33 0x2E 0x6F 0x72 0x67
0x2F 0x31 0x39 0x39 0x39 0x2F 0x30 0x32 0x2F 0x32 0x32 0x2D 0x72 0x64
0x66 0x2D 0x73 0x79 0x6E 0x74 0x61 0x78 0x2D 0x6E 0x73 0x23
("<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#</code>"),
increment <var>s</var> by 24 and enter loop <var>N</var>:

<ol>
<li>
If <var>sequence</var>[<var>s</var>] is undefined, the
<a>computed MIME type</a> is the <a>supplied MIME
type</a>.

Abort these steps.

<li>
If <var>length</var> is greater than or equal to <var>s</var> + 43
and the forty-three <a>bytes</a> from
<var>sequence</var>[<var>s</var>] to
<var>sequence</var>[<var>s</var> + 42] are equal to 0x68 0x74 0x74
0x70 0x3A 0x2F 0x2F 0x70 0x75 0x72 0x6C 0x2E 0x6F 0x72 0x67 0x2F
0x72 0x73 0x73 0x2F 0x31 0x2E 0x30 0x2F
("<code>http://purl.org/rss/1.0/</code>"), the <a>computed
MIME type</a> is "<code>application/rss+xml</code>".

Abort these steps.

<li>
Increment <var>s</var> by 1.
</ol>

<li>
Increment <var>s</var> by 1.
</ol>

<li>
The <a>computed MIME type</a> is the <a>supplied MIME
type</a>.

Abort these steps.
</ol>
</ol>

<li>
The <a>computed MIME type</a> is the <a>supplied MIME type</a>.
</ol>

<p class=note>
It might be more efficient for the user agent to implement the <a>rules
for distinguishing if a resource is a feed or HTML</a> in parallel with
its algorithm for detecting the character encoding of an HTML document.



<h2 id=context-specific-sniffing>Context-specific sniffing</h2>

<p class=XXX>
Expand Down

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