Gecko allows users to type both styles if caret is moved from which style. Therefore, this behavior is intentional at least when this is designed. (Instead, Blink does not allow users to put to start of following text node, therefore, the previous style is always used.) In the editing API working group, this kind of Blink's limitation should be fixed because of this does not allow web apps to put caret with Selection API and user types in specific styled element. Therefore, we should be careful about that how they will fix this issue. On the other hand, I do the operation in Word, I see same result as Blink. So, it may be worthwhile to change the caret position at joining the paragraphs to make the previous paragraph's style be preferred.
Bug 1808722 Comment 2 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Gecko allows users to type both styles at the style boundary if caret is moved from either style. Therefore, this behavior is intentional at least when this is designed. (Instead, Blink does not allow users to put to start of following text node, therefore, the previous style is always used.) In the editing API working group, this kind of Blink's limitation should be fixed because of this does not allow web apps to put caret with Selection API and user types in specific styled element. Therefore, we should be careful about that how they will fix this issue. On the other hand, I do the operation in Word, I see same result as Blink. So, it may be worthwhile to change the caret position at joining the paragraphs to make the previous paragraph's style be preferred.