Vertical text support is coming on quickly in Firefox. There’s still a bunch of blockers, but they’re being knocked off fast: https://bugzilla.mozilla.org/show_bug.cgi?id=145503

That's great to see!

the syntax is really confusing though. Duplicating characters? I thought it was hard to follow what would end up where https://hacks.mozilla.org/2015/03/ruby-support-in-firefox-de...

The syntax isn't that bad. It's written in a style that gracefully degrades: strip out the HTML tags, and it's perfectly readable, but the furigana shows up inline rather than above.

Portions are duplicated to ensure graceful degradation. You type 振り仮名(ふりがな) (with tags demarcating the sections, of course), repeating the り, so it'll show up properly if the browser lacks ruby text support. But if the browser does have ruby text support, it can hide the り from the furigana because it's redundant:

  振 仮名
  ふりがな

On iPad the り is only visible in the furigana. On top of that, the furigana is displayed below the text.

Yes, only Firefox 38 has support for advanced ruby text, unfortunately.

While advanced ruby text degrades great in browsers with no ruby text support at all, it doesn't seem to degrade well in browsers with basic support :(

Oh wow, I screwed that up. I meant to type this:

  ふ　がな
  振り仮名

Furigana is a pronunciation guide for kanji, not the other way around. ;)