[css-text] Writing System prose is currently unimplementable on ICU #4445
Description
The spec says:
While most languages have a preferred writing system, many can also be transcribed into a different writing system. As a common example, most languages have at least one Latin transcription, and can thus be written in the Latin writing system. In these cases the document typically adopts the typographic conventions of the Latin writing system: for example Japanese “romaji” and Chinese Pinyin use Latin letters and word spaces, and follow Latin line-breaking and justification practices accordingly. As another example, historical ideographic Korean (ko-Hani) does not use word spaces, and should therefore be typeset as for Chinese.
The Web Platform Test css/css-text/writing-system/writing-system-line-break-001.html tests this, by setting lang=ja-Latn on an element and checking line breaking behavior. ICU does not abide by the above quoted prose, and erroneously uses Japanese-style line breaking.