It turns out that WebKit has been shipping a prohibition against breaks before small kana for awhile. But only sometimes: it depends on whether the language is tagged or not, see testcase. Tagged content is looser, untagged content is stricter. Firefox afaict never allows breaks before small kana.

I think this opens up the question of, should we reconsider defaulting line-break: normal to prohibiting breaks before small kana? Since it seems to be Web-compatible to change, would that be more natural for Japanese?