There is test files as testing/web-platform/tests/css/css-text/i18n/<lang>/css-text-line-break-<lang>-pr-<line break value>.html. But all tests is incorrect according to looking test and spec.

From https://w3c.github.io/csswg-drafts/css-text/#line-break-property

The following breaks are allowed for loose if the writing system is Chinese or Japanese and are otherwise forbidden:
...
breaks after prefixes:
Characters with the Unicode line breaking class PR [UAX14] and the East Asian Width property [UAX11] Ambiguous, Fullwidth, or Wide.

But https://searchfox.org/mozilla-central/rev/9de332d5c8faac58dc1232b8a6383ce6cb1400f4/testing/web-platform/tests/css/css-text/i18n/ja/css-text-line-break-ja-pr-loose.html#10

When the language is Japanese, and line-break:loose, a browser allows a break before a PR character with East Asian Width of A, F, or W.

So it means that this tests is incorrect that is before vs after.

According to https://w3c.github.io/csswg-drafts/css-text/#line-break-property,

The following breaks are allowed for loose if the writing system is Chinese or Japanese and are otherwise forbidden:
...
breaks after prefixes:
Characters with the Unicode line breaking class PR [UAX14] and the East Asian Width property [UAX11] Ambiguous, Fullwidth, or Wide.

But in testing/web-platform/tests/css/css-text/i18n/ja/css-text-line-break-ja-pr-loose.html,

When the language is Japanese, and line-break:loose, a browser allows a break before a PR character with East Asian Width of A, F, or W.

It means that many tests for PR property are incorrect (It mistakes before and after). We should update it to match spec.

NI for updating the patch to address Jonathan's review comments when you have the cycle. Thanks!