Summary:
The 'prefers-color-scheme' media feature is way for pages to detect
if the user prefers a light or dark color theme. The values are
'light', 'dark', and 'no-preference'. If the "resist fingerprinting"
feature is active we always match 'light'. If the media type is
'print' we always match 'light'. Otherwise, we try to determine
a value from the user's current "desktop theme". This should work
well on recent versions of OSX and Windows. On Linux, we don't
know how to determine a value from the system so we match
'no-preference' there - help wanted:
https://bugzilla.mozilla.org/show_bug.cgi?id=1525775
Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1494034
Link to standard:
https://drafts.csswg.org/mediaqueries-5/#prefers-color-scheme
Platform coverage: All platforms
Estimated or target release: Firefox 67
Preference behind which this will be implemented:
The feature is always enabled, but there is an existing hidden
preference to set a value (integer), ui.systemUsesDarkTheme:
0 = light
1 = dark
2 = no-preference
Is this feature enabled by default in sandboxed iframes?
Yes, but if widget look-and-feel features are disabled in
sandboxed iframes then we'll match 'no-preference'.
DevTools bug: none
Do other browser engines implement this?
Safari has implemented this feature and it's available in their
Nightly build, but it's not yet shipped AFAICT.
https://paulmillr.com/posts/using-dark-mode-in-css/ is a post about
implementation of this feature in Safari;
https://webkit.org/blog/8475/release-notes-for-safari-technology-preview-68/
documents its addition.
It appears Chrome is actively working on it:
https://bugs.chromium.org/p/chromium/issues/detail?id=889087
web-platform-tests:
https://w3c-test.org/css/mediaqueries/prefers-color-scheme.html
Is this feature restricted to secure contexts? No
Credit also goes to Jonathan Kingston who wrote the initial
implementation of this feature.
/Mats
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