Dear maintainers:

I found eww fails to show card-images which is described in the following
web page:

https://www.geeksforgeeks.org/why-card-images-use-data-src-not-src-for-image-in-bootstrap-4/

For example, if you open following page with eww & firefox, you will notice
the differences:   http://www.interdb.jp/pg/pgsql05.html

I modified shr.el a little bit to support "data-src" attribute, so eww can
display card-images properly.

 Hope it helps.

Regards
yc