Document that the Japanese calendar is currently broken in browsers for dates before 1868-10-23 (#41420)

* Document that the Japanese calendar is currently broken in browser for dates before 1868-10-23

* Discuss dates before 1868-10-23 after the table

* Elaborate even more

* Say which calendar the remarks are about

* Tweak wording

* Suggested edits

Co-authored-by: Joshua Chen <[email protected]>

* Update files/en-us/web/javascript/reference/global_objects/intl/supportedvaluesof/index.md

* Update files/en-us/web/javascript/reference/global_objects/intl/supportedvaluesof/index.md

---------

Co-authored-by: Joshua Chen <[email protected]>

Author: hsivonen

files/en-us/web/javascript/reference/global_objects/intl/supportedvaluesof/index.md

@@ -76,10 +76,12 @@ Below are all values that are commonly supported by browsers for the `calendar`
 | `islamic-tbla`     | Hijri calendar, tabular/rule-based with leap year rule II (leap years 2,5,7,10,13,16,18,21,24,26,29 in the 30-year cycle (1-based numbering)) and Thursday/astronomical epoch (July 15, 622 Julian / 0622-07-18 ISO)                                                                                                                                             |
 | `islamic-civil`    | Hijri calendar, tabular/rule-based with leap year rule II (leap years 2,5,7,10,13,16,18,21,24,26,29 in the 30-year cycle (1-based numbering)) and Friday/civil epoch (July 16, 622 Julian / 0622-07-19 ISO)                                                                                                                                                      |
 | `iso8601`          | ISO calendar (variant of the Gregorian calendar with week rules and formatting parameters made region-independent)                                                                                                                                                                                                                                               |
-| `japanese`         | Japanese Imperial calendar (this calendar adds an era for each new emperor, so the output year and era for a future date may not match the input year and era when your code runs on a future engine version)                                                                                                                                                    |
+| `japanese`         | Japanese Imperial calendar (this calendar adds an era for each new emperor, so the output year and era for a future date may not match the input year and era when your code runs on a future engine version. **Note:** See the remarks below this table about dates prior to 1868-10-23 ISO.)                                                                   |
 | `persian`          | Persian calendar                                                                                                                                                                                                                                                                                                                                                 |
 | `roc`              | Republic of China calendar                                                                                                                                                                                                                                                                                                                                       |
 
+As of October 2025, in the `japanese` calendar, dates prior to 1868-10-23 ISO (the start date of the year 1 Meiji) don't work as expected in browsers in two ways. First, [CLDR had the wrong start date for the Meiji era](https://unicode-org.atlassian.net/browse/CLDR-11375), which causes calendar implementations to extend the Meiji era further to the past than it actually did. Second, the upcoming [Intl era and monthCode Proposal](https://tc39.es/proposal-intl-era-monthcode/) specifies that dates prior to the Meiji era should use Gregorian eras, but browsers have traditionally used approximations of prior Japanese eras instead. The `japanese` calendar was taken into use on January 1, 6 Meiji / 1873-01-01 ISO, so these problems only affect proleptic dates.
+
 The types below are specified in CLDR but do not have implementations distinct from the above calendars in browsers.
 
 | Value                            | Description                           | Notes                                                                                                                                                                                                                                   |