I have a folder of 400+ photos to which I have added captions on my iPhone, and they show there, but I see no way to display the caption through the web app. Below them they actually show "untitled".
You are seeing "untitled" below the thumbnails, because Photos on www.icloud.com is showing the titles below the thumbnails, but not the caption. As Photos on the iPhone can only add a caption, but not a title, you are out of luck. In Photos for Mac you can add both - a caption and a title. It is a bit silly, but I have to add the descriptions twice on my Mac, once as the title, and once as the caption. Only this way I can see them below the thumbnails in Photos for Mac and on www.icloud.com and as a caption on my iPhone and iPad.
That functionality is not included in the online web app.
no option to show photo captions on iCloud.com Photos app, show as "Untitled"
