This appears to be a spec issue, with Firefox and Chrome going with the SVG spec and Edge going with what is probably a more sensible interpretation.

Could you please raise the issue with the SVG spec working group to get the intention clarified? We will follow their lead.

Let us know if you cannot file a spec issue for some reason.

Chrome and Firefox are not following the spec. BCP 47 says "en" and "EN" are the same language language, so they are equal.

The SVG spec refers to BCP 47. BCP 47 says:

"All comparisons MUST be performed in a case-insensitive manner."

"A language range matches a particular language tag if, in a case-insensitive comparison, it exactly equals the tag, or if it exactly equals a prefix of the tag such that the first character following the prefix is "-"."

"The language range is compared, in a case-insensitive manner, to each language tag being matched, using basic string processing."

How can Chrome be spec compliant when it is doing case sensitive comparisons?

If you look at Chrome handling CSS psuedo class :lang() selectors, :lang(de) matches lang="de" matches lang="DE" and even lang="De". CSS is using case-insensitive matching.

Therefore, Chrome is currently inconsistent with its treatment of language tag matching. It matches them correctly for :lang() and incorrectly for systemLanguage.

The intention of W3C is clear. The lang and xml:lang attributes are valid Language Tags. Capitalization does not count in language tags.

That Firefox also messes up is not determinative of anything.

Pointing out an internal inconsistency in CHrome is helpful, thanks.

I agree that SVG spec should be worded more carefully, so I would like the SVG working group to fix that wording. Meanwhile, we can change to match your requested behavior, I think.

Hello,
Can I start working on it and try to fix this bug?

Sure, work on it if you would like. I can help you through the process if necessary.

Yes, this bug should be an easy fix. Either comparisons should be made case insensitive or (if langtags are being used as keys) the langtags should be lowercased (canonical case).

It should also be easy to fix https://bugs.chromium.org/p/chromium/issues/detail?id=732063 , too. Chrome treats the systemLanguage attribute as a space separated list (just like the class attribute), but the specification says it is a comma separated list.

https://www.w3.org/TR/SVG11/struct.html#ConditionalProcessingSystemLanguageAttribute

"The attribute value is a comma-separated list of language names as defined in BCP 47 [BCP47]."

The comma-separation aspect would need to be fixed both on input (parsing) and output (serialization).

Thanks, I start working on both bugs.

There's another systemLanguage bug in Chrome: it does not do systemLanguage prefix matching correctly. If the user preference is "en", then Chrome should match systemLanguage="en-US-kumquat", but it does not.

I opened https://bugs.chromium.org/p/chromium/issues/detail?id=872378

Firefox nightlies pass now.

I'm amused, and I love it. Speedy off-site service.

I came across this bug in Chrome and Firefox, so I reported it here on 31 July. I would have reported it at Bugzilla, but I don't have an account there.

A few days later, a bug report with the same jsfiddle appears at https://bugzilla.mozilla.org/show_bug.cgi?id=1480946 .

A week later, Firefox is fixed.

The NextAction date has arrived: 2018-08-13

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59

commit 1f24b02c1e563e405a59d5d44eeb03a6f2c00d59
Author: Fredrik Söderquist <fs@opera.com>
Date: Thu Aug 30 15:29:57 2018

Fix SVG systemLanguage conditional processing

The algorithm that performs the test between browser language preference
and the systemLanguage value has been modified
to implement a language-tag match based on the prefix,
making it consistent with BCP 47's basic filter operation and
as specified in the SVG 1.1 spec.

The previous behavior was:

- Tests whether the value of the attribute (e.g., "en-us") is a prefix
of the user preferred language (e.g., "en")
(which would evaluate to false)

The test was modified to compare in the opposite manner,
now user preferred language must be a prefix of the value.

- If the language tag length was not 2, the language tag
was not matched, (e.g. It did not match "en-us" to "en-us").
This check has been removed.

- The algorithm did not check for a trailing hyphen ("-").
We do not want langtag "it" matching langtag "ita",
but it is ok for "it" to match "it-it".
Now it does.

------------

The algorithm that performs the test between browser language preference
and the systemLanguage value has been modified
to implement a case insensitive language-tag match:

"en" matches "EN".

The previous behavior was case sensitive.

------------

Add Luca Di Domenico to AUTHORS

------------

Bug:  872378 ,  869440 
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ibe767644b5a92e8d79ffe4b7777f5f51851b3883
Reviewed-on: https://chromium-review.googlesource.com/1188314
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#587592}
[modify] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/AUTHORS
[modify] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/third_party/WebKit/LayoutTests/platform/linux/svg/W3C-SVG-1.1/struct-cond-02-t-expected.png
[modify] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/third_party/WebKit/LayoutTests/platform/linux/svg/W3C-SVG-1.1/struct-cond-02-t-expected.txt
[modify] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/third_party/WebKit/LayoutTests/platform/mac/svg/W3C-SVG-1.1/struct-cond-02-t-expected.png
[modify] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/third_party/WebKit/LayoutTests/platform/mac/svg/W3C-SVG-1.1/struct-cond-02-t-expected.txt
[modify] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/third_party/WebKit/LayoutTests/platform/win/svg/W3C-SVG-1.1/struct-cond-02-t-expected.png
[modify] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/third_party/WebKit/LayoutTests/platform/win/svg/W3C-SVG-1.1/struct-cond-02-t-expected.txt
[add] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/third_party/WebKit/LayoutTests/svg/dom/systemLanguage-case-insensitive.html
[add] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/third_party/WebKit/LayoutTests/svg/dom/systemLanguage-langtag-match-prefix.html
[modify] https://crrev.com/1f24b02c1e563e405a59d5d44eeb03a6f2c00d59/third_party/blink/renderer/core/svg/svg_tests.cc