Line-breaks are possible within elements with `display: ruby`. A single pair of a ruby-base and a ruby-text has never been line-breakable, and it has been pushed to the next line if the current line had no enough space for the entire pair. Now each of the ruby-base and the ruby-text can be split into multiple lines.
Text layout with <ruby> was not great. It's worse if there are long rubies or users browse pages in smaller screens such as mobile devices.
Docs: https://docs.google.com/document/d/1hzvrwoE_0aw08X_CaU40zV5bXbMQjY2SHQHj3Np4sDo/edit?usp=sharing