Why do SRT subtitles fail to appear in my MP4 videos on iPhones?
Update with answer

It can be done. Run your source files through Handbrake, after importing the subtitle files you want. Then export as Mp4. I've read (a) you should put the SRT file/s in the same folder as the Mp4 on your server, and (b) the SRT should have the same name as the mp4. But (a) is probably wrong (in a text-editor, you can see the subtitles have been coded into the mp4 by Handbrake) and (b) is definitely wrong - two different language SRT files will obviously have different file names.  

Testing... On iPhone, a little speech bubble appeared on the right side of the QT player control bar when it loaded my MP4. Tapping that popped up the two subtitle language options. So, it worked. But it only worked because first my tester had to enable "closed captions" in his iPhone's:

Settings &gt; General &gt; Accessibility &gt; Subtitles &amp; Captioning &gt; Closed Captions + SDH


In other words, an extra chore for users and the result is crappy-looking subtitles anyway. And all this because, unlike Samsungs and most other mobiles I've tested on, the iPhone overrides the videojs player and plays your videos in Apple's own Quicktime player.