Backed out changeset 088b1611348a (bug 1841117) as requested by canadahonk CLOSED TREE

docs/code-quality/coding-style/css_guidelines.rst

@@ -259,6 +259,12 @@ Text Direction
    Values <https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Logical_Properties>`__
    for more information.
 
+Testing
+~~~~~~~
+
+To test for RTL layouts, you can go to ``about:config`` and set
+``intl.uidirection`` to ``-1``.
+
 Writing cross-platform CSS
 --------------------------
 

modules/libpref/init/all.js

@@ -1591,6 +1591,8 @@ pref("intl.regional_prefs.use_os_locales",  false);
 pref("font.language.group",                 "chrome://global/locale/intl.properties");
 pref("font.cjk_pref_fallback_order",        "zh-cn,zh-hk,zh-tw,ja,ko");
 
+pref("intl.uidirection", -1); // -1 to set from locale; 0 for LTR; 1 for RTL
+
 // This pref controls pseudolocales for testing localization.
 // See https://firefox-source-docs.mozilla.org/l10n/fluent/tutorial.html#manually-testing-ui-with-pseudolocalization
 pref("intl.l10n.pseudo", "");