The title is a little click-baity, I think they're really just saying current browsers don't handle subtitles adequately and should be improved. As in, "if you can read this, your browser captions are broken, because all browser captions are broken".

There's also some browser-specific bugs mentioned in the article, but I don't think those are really what the title is about.

I think it's referring to the Safari bug they mention, where the cue gets readded without the old one being removed.