On Thu, 21 Jul 2016 17:46:18 +0900, Katsumi Yamaoka wrote:
> I realized `+' works for some html mails but doesn't for others.

I'm not quite sure this is the right approach, but I seem to have
found the cause and the solution.  `shr-image-fetched', that
`shr-tag-img' calls, overrides the keymap text property on an
image with the value that was there before inserting the image,
that is, nil.  Could you please test this patch?