This is a follow up to bug#22547 which seems to be fixed for the
original test cases but is not fixed in general.

When reading mail/news in Gnus, HTML messages sometimes have patches
of missing background colour. That is, there are patches where the
window background colour shows through. This problem is not apparent
in Emacs 24.

The problem does not seem to happen where there is text in the
foreground. Rather, the HTML background colour is missing on some
lines (before and) after the text (begins or) ends.

mm-text-html-renderer is `shr' (default). Also, possibly relevant,
in Customize I have:

 '(shr-color-visible-luminance-min 75 t)
 '(shr-use-fonts nil)

Sreenshots and raw text of two new problematic messages are
attached.